【Codeforces 815C】Karen and Supermarket

相关链接

题目传送门:http://codeforces.com/contest/815/problem/C

解题报告

这题就是考察树DP优化复杂度的一种常用技巧
考虑暴力DP的话,复杂度是:$O(n^3)$的
但如果在父亲结点那里记录一个儿子节点的子树大小的前缀和,复杂度就变成了$O(n^2)$
证明也很简单,对于任意两个结点,只会在$LCA$处产生$1$的花费

Code

#include<bits/stdc++.h>
#define LL long long
using namespace std;

const int N = 5009;
const LL INF = 1e14;

int n, head[N], nxt[N], to[N], sz[N];
LL b, f[N][N], g[N][N], c[N], d[N], t1[N], t2[N];

inline int read() {
	char c=getchar(); int ret=0,f=1;
	while (c<'0'||c>'9') {if(c=='-')f=-1;c=getchar();}
	while (c<='9'&&c>='0') {ret=ret*10+c-'0';c=getchar();}
	return ret*f;
}

inline void AddEdge(int u, int v) {
	static int E = 1;
	to[++E] = v; nxt[E] = head[u]; head[u] = E;	
}

inline void relax(LL &a, LL b) {
	a = a > b? b: a;
}

inline void DFS(int w) {
	f[w][0] = g[w][0] = 0;
	for (int i = head[w]; i; i = nxt[i]) {
		DFS(to[i]);
		memcpy(t1, f[w], sizeof(t1));
		memcpy(t2, g[w], sizeof(t2));
		for (int j = 0; j <= sz[w]; j++) {
			for (int k = 0; k <= sz[to[i]]; k++) {
				relax(f[w][j + k], t1[j] + f[to[i]][k]);
				relax(g[w][j + k], t2[j] + g[to[i]][k]);
			}
		}
		sz[w] += sz[to[i]];
	}
	sz[w]++;
	for (int i = sz[w] - 1; ~i; i--) {
		g[w][i + 1] = g[w][i] + c[w] - d[w];
		relax(f[w][i + 1], f[w][i] + c[w]);
		relax(g[w][i + 1], f[w][i + 1]);
	}
	g[w][0] = 0;
}

int main() {
	n = read(); b = read();
	c[1] = read(); d[1] = read();
	for (int i = 2; i <= n; i++) {
		c[i] = read(); d[i] = read();
		AddEdge(read(), i);
	}
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= n; j++) {
			f[i][j] = g[i][j] = INF;
		}
	}
	DFS(1);
	for (int i = n; ~i; i--) {
		if (g[1][i] <= b) {
			printf("%d\n", i);
			exit(0);
		}
	}
	return 0;
}

【Codeforces 817E】Choosing The Commander

相关链接

题目传送门:http://codeforces.com/contest/817/problem/E

解题报告

考虑异或之后小于的话
把士兵插入到Trie树里面,然后走一条链即可
时间复杂度:$O(n \log 10^8)$

Code

#include<bits/stdc++.h>
#define LL long long
using namespace std;

const int N = 100009 * 32;

inline int read() {
	char c = getchar();
	int ret = 0, f = 1;
	while (c < '0' || c > '9') {
		f = c == '-'? -1: 1;
		c = getchar();
	}
	while ('0' <= c && c <= '9') {
		ret = ret * 10 + c - '0';
		c = getchar();
	}
	return ret * f;
}

class Trie{
int root, cnt;
struct Node{
	int sum, ch[2];
}p[N];
public:
	inline void modify(int v, int d) {
		modify(root, 30, v, d);
	}
	inline int query(int buf, int lim) {
		return query(root, 30, buf, lim);
	}
private:
	inline void modify(int &w, int t, int v, int d) {
		w = w? w: ++cnt;
		p[w].sum += d;
		if (t >= 0) {
			modify(p[w].ch[(v >> t) & 1], t - 1, v, d);
		}
	}
	inline int query(int w, int t, int buf, int lim) {
		if (!w || t == -1) {
			return 0;
		} else {
			int ret = 0, t2 = (buf >> t) & 1, t1 = (lim >> t) & 1;
			if (t1) {
				ret += p[p[w].ch[t2]].sum;
				ret += query(p[w].ch[t2 ^ 1], t - 1, buf, lim);
			} else {
				ret += query(p[w].ch[t2], t - 1, buf, lim);
			}
			return ret;
		}
	}
}trie;

int main() {
	int n = read();
	for (int i = 1; i <= n; i++) {
		int ty = read();
		if (ty == 1) {
			trie.modify(read(), 1);
		} else if (ty == 2) {
			trie.modify(read(), -1);
		} else {
			int p = read(), l = read();
			printf("%d\n", trie.query(p, l));
		}
	}
	return 0;
}

【Codeforces 212B】Polycarpus is Looking for Good Substrings

相关链接

题目传送门:http://codeforces.com/contest/212/problem/B
中文题面:http://www.tsinsen.com/A1470

解题报告

这题你需要观察到一个非常重要的结论:

以字符串某个特定位置为开头的字符串,至多只有26个会产生贡献

于是我们就可以暴力枚举这$O(26n)$的字符串
然后去更新答案
时间复杂度:$O(26n \log q)$

Code

这份代码我写挫了
时间复杂度是:$O(676n \log q)$的

#include<bits/stdc++.h>
#define LL long long
using namespace std;

const int N = 1000009;
const int SGZ = 26;
const int M = 10009;

int n, m, nxt[N][SGZ], crt[N][SGZ], q[M], ans[M];
vector<int> val;
char s[N], qy[SGZ]; 

inline int read() {
	char c = getchar();
	int ret = 0, f = 1;
	while (c < '0' || c > '9') {
		f = c == '-'? -1: 1;
		c = getchar();
	}
	while ('0' <= c && c <= '9') {
		ret = ret * 10 + c - '0';
		c = getchar();
	}
	return ret * f;
}

inline int index(int x) {
	for (int l = 0, r = val.size() - 1, mid; l <= r; ) {
		mid = l + r >> 1;
		if (val[mid] == x) {
			return mid; 
		} else if (val[mid] < x) {
			l = mid + 1;
		} else {
			r = mid - 1;
		}
	} 
	return -1;
}

int main() {
	scanf("%s", s);
	n = strlen(s);
	m = read();
	for (int i = 1; i <= m; i++) {
		scanf("%s", qy);
		int len = strlen(qy);
		for (int j = 0; j < len; j++) {
			q[i] |= 1 << qy[j] - 'a';
		}
		val.push_back(q[i]);
	}
	sort(val.begin(), val.end());
	val.resize(unique(val.begin(), val.end()) - val.begin());
	int last_position[SGZ];
	fill(last_position, last_position + SGZ, n);
	for (int i = n - 1; ~i; i--) {
		last_position[s[i] - 'a'] = i;
		for (int j = 0; j < SGZ; j++) {
			nxt[i][j] = last_position[j];
		}
	}
	memset(crt, -1, sizeof(crt));
	for (int i = 0; i < n; i++) {
		for (int j = 0, p = i, sta = 0; j < 26 && p < n; j++) {
			int np = n, c = -1;
			for (int k = 0; k < 26; k++) {
				if ((sta >> k & 1) == 0) {
					if (np > nxt[p][k]) {
						c = k;
						np = nxt[p][k];
					}
				}
			}
			if (~c) {
				sta |= 1 << c;
				p = np;
				crt[i][j] = sta;
				if (!i || crt[i - 1][j] != sta) {
					int idx = index(sta);
					if (~idx) { 
						ans[idx]++;
					}
				}
			}
		}
	}
	for (int i = 1; i <= m; i++) {
		printf("%d\n", ans[index(q[i])]);
	}
	return 0;
}

【Codeforces 734F】Anton and School

相关链接

题目传送门:http://codeforces.com/contest/734/problem/F
官方题解:http://codeforces.com/blog/entry/48397

解题报告

画一画维恩图便容易发现:$(a \ and \ b) + (a \ or \ b) = a + b$
所以$b_i + c_i = \sum\limits_{j = 1}^{n}{a_i + a_j} = na_i + \sum\limits_{j = 1}^{n}{a_j}$
于是有$\sum\limits_{i = 1}^{n}{a_i} = \frac{\sum\limits_{i = 1}^{n}{b_i + c_i}}{2n}$
最后加加减减搞一搞就可以求出$a_i$了

然后就是检查${a_i}$是否符合${b_i}$和${c_i}$
这个按二进制位枚举一下就可以了
总的时间复杂度是:$O(n \log \max(a_i))$的

Code

#include<bits/stdc++.h>
#define LL long long
using namespace std;

const int N = 200009;

int n, a[N], b[N], c[N];

inline int read() {
	char c = getchar();
	int ret = 0, f = 1;
	while (c < '0' || c > '9') {
		f = c == '-'? -1: 1;
		c = getchar();
	}
	while ('0' <= c && c <= '9') {
		ret = ret * 10 + c - '0';
		c = getchar();
	}
	return ret * f;
}

inline bool check() {
	static int cnt[100];
	for (int i = 1; i <= n; i++) {
		for (int t = 0, v = a[i]; v; v >>= 1, ++t) {
			cnt[t] += v & 1;
		}
	}
	for (int i = 1; i <= n; i++) {
		int tb = 0, tc = 0;
		for (int j = 0, cur = 1; j <= 30; j++, cur <<= 1) {
			tb += (a[i] & cur)? cnt[j] * cur: 0;
			tc += (a[i] & cur)? n * cur: cnt[j] * cur;
		}
		if (tb != b[i] || tc != c[i]) {
			return false;
		}
	}
	return true;
}

int main() {
	n = read();
	for (int i = 1; i <= n; i++) {
		a[i] = b[i] = read();
	}
	LL sum = 0;
	for (int i = 1; i <= n; i++) {
		a[i] += (c[i] = read());
		sum += a[i];
	}
	if (sum % (n << 1)) {
		puts("-1");
		exit(0);
	}
	sum /= n << 1;
	for (int i = 1; i <= n; i++) {
		if ((a[i] -= sum) % n) {
			puts("-1");
			exit(0);
		} else {
			a[i] /= n;
		}
	}
	if (!check()) {
		puts("-1");
		exit(0);
	} 
	for (int i = 1; i <= n; i++) {
		printf("%d ", a[i]);
	}
	return 0;
}

【日常小测】魔术卡

相关链接

题目传送门:http://oi.cyo.ng/wp-content/uploads/2017/06/20170614-statement.pdf

题目大意

给你$m(m \le 10^3)$种,第$i$种有$a_i$张,共$n(n = \sum\limits_{i = 1}^{m}{a_i} \le 5000)$张卡
现在把所有卡片排成一排,定义相邻两个卡片颜色相同为一个魔术对
询问恰好有$k$个魔术对的本质不同的排列方式有多少种,对$998244353$取模
定义本质不同为:至少有一位上的颜色不同

解题报告

一看就需要套一个广义容斥原理
于是问题变为求“至少有$x$个魔术对的方案数”

于是我们可以钦定第$i$种卡片组成了$j$个魔术对
然后用一个$O(n^2)$的$DP$来求出至少有$x$个魔术对的方案数

为了方便去重,我们先假设相同颜色的卡片有编号,最后再依次用阶乘除掉
考试的时候就是这里没有处理好,想的是钦定的时候直接去重,但这样块与块之间的重复就搞不了,于是$GG$了

Code

#include<bits/stdc++.h>
#define LL long long
using namespace std;

const int N = 5009;
const int MOD = 998244353;

int n, m, K, a[N], pw[N], inv[N], f[N][N], C[N][N];

inline int read() {
	char c = getchar();
	int ret = 0, f = 1;
	while (c < '0' || c > '9') {
		f = c == '-'? -1: 1;
		c = getchar();
	}
	while ('0' <= c && c <= '9') {
		ret = ret * 10 + c - '0';
		c = getchar();
	}
	return ret * f;
}

inline int Pow(int w, int t) {
	int ret = 1;
	for (; t; t >>= 1, w = (LL)w * w % MOD) {
		if (t & 1) {
			ret = (LL)ret * w % MOD;
		}
	}
	return ret;
}

int main() {
	freopen("magic.in", "r", stdin);
	freopen("magic.out", "w", stdout);
	m = read(); n = read(); K = read();
	for (int i = 1; i <= m; i++) {
		a[i] = read();
	}
	C[0][0] = 1;
	for (int i = 1; i <= n; i++) {
		C[i][0] = 1;
		for (int j = 1; j <= n; j++) {
			C[i][j] = (C[i - 1][j] + C[i - 1][j - 1]) % MOD;
		}
	}
	pw[0] = inv[0] = 1;
	for (int i = 1; i <= n; i++) {
		pw[i] = (LL)pw[i - 1] * i % MOD;
		inv[i] = Pow(pw[i], MOD - 2);
	}
	f[0][0] = 1;
	for (int i = 1, pre_sum = 0; i <= m; i++) {
		pre_sum += a[i] - 1;
		for (int j = 0; j <= pre_sum; j++) {
			for (int k = min(a[i] - 1, j); ~k; k--) {
				f[i][j] = (f[i][j] + (LL)f[i - 1][j - k] * C[a[i]][k] % MOD * pw[a[i] - 1] % MOD * inv[a[i] - 1 - k]) % MOD;
			}
		} 
	}
	int ans = 0;
	for (int i = K, ff = 1; i < n; i++, ff *= -1) {
		f[m][i] = (LL)f[m][i] * pw[n - i] % MOD;
		ans = (ans + (LL)ff * C[i][K] * f[m][i]) % MOD;
	}
	for (int i = 1; i <= m; i++) {
		ans = (LL)ans * inv[a[i]] % MOD;
	}
	printf("%d\n", (ans + MOD) % MOD);
	return 0;
}

【Codeforces 736C】Ostap and Tree

相关链接

题目传送门:http://codeforces.com/contest/736/problem/C
官方题解:http://codeforces.com/blog/entry/48659

解题报告

就简单DP一下
记录子树中最深的、还未与黑点匹配的点
记录可以子树中的黑点可以向上覆盖多少
时间复杂度:$O(nm^2)$

Code

#include<bits/stdc++.h>
#define LL long long
#define relax(a, b, c) (a = (a + (LL)b * c) % MOD)
using namespace std;

const int N = 300;
const int MOD = 1000000007;
const int BAS = 120;

int n, K, head[N], nxt[N], to[N];
int *f[N], arr_back[N][N];

inline int read() {
	char c = getchar();
	int ret = 0, f = 1;
	while (c < '0' || c > '9') {
		f = c == '-'? -1: 1;
		c = getchar();
	}
	while ('0' <= c && c <= '9') {
		ret = ret * 10 + c - '0';
		c = getchar();
	}
	return ret * f;
}

inline void AddEdge(int u, int v) {
	static int E = 1;
	to[++E] = v; nxt[E] = head[u]; head[u] = E;
	to[++E] = u; nxt[E] = head[v]; head[v] = E;
}

inline void DFS(int w, int fa) {
	static int arr1_back[N], *tmp = arr1_back + BAS;
	f[w][-1] = f[w][K] = 1;
	for (int i = head[w], t; i; i = nxt[i]) {
		if ((t = to[i]) != fa) {
			DFS(t, w);
			for (int j = -K; j <= K; j++) {
				tmp[j] = f[w][j];
				f[w][j] = 0;
			}
			for (int j = -K; j <= K; j++) {
				for (int k = -K - 1; k < K; k++) {
					if (j >= 0 && k >= 0) {
						relax(f[w][max(j, k)], tmp[j], f[t][k + 1]);	
					} else if ((j < 0 && k >= 0) || (j >= 0 && k < 0)) {
						int ge = max(j, k), le = min(j, k);
						if (ge + 1 >= -le) {
							relax(f[w][ge], tmp[j], f[t][k + 1]);	
						} else {
							relax(f[w][le], tmp[j], f[t][k + 1]);
						}
					} else {
						relax(f[w][min(j, k)], tmp[j], f[t][k + 1]);
					}
				}
			}
		}
	}
}

int main() {
	n = read(); K = read();
	for (int i = 1; i < n; i++) {
		AddEdge(read(), read());
	}
	for (int i = 1; i <= n; i++) {
		f[i] = arr_back[i] + BAS;
	}
	DFS(1, 1);
	int ans = 0;
	for (int i = 0; i <= K; i++) {
		ans = (ans + f[1][i]) % MOD;
	} 
	printf("%d\n", ans);
	return 0;
}

【BZOJ 3884】上帝与集合的正确用法

相关链接

题目传送门:http://www.lydsy.com/JudgeOnline/problem.php?id=3884
官方题解:http://blog.csdn.net/popoqqq/article/details/43951401

解题报告

根据欧拉定理有:$a^x \equiv a^{x \% \varphi (p) + \varphi (p)} \mod p$
设$f(p)=2^{2^{2 \cdots }} \mod p$
那么有$f(p) = 2^{f(\varphi(p)) + \varphi(p)} \mod p$

如果$p$是偶数,则$\varphi(p) \le \frac{p}{2}$
如果$p$是奇数,那么$\varphi(p)$一定是偶数
也就是说每进行两次$\varphi$操作,$p$至少除$2$
所以只会递归进行$\log p$次
总时间复杂度:$O(T\sqrt{p} \log p)$

Code

#include<bits/stdc++.h>
#define LL long long
using namespace std;

inline int read() {
	char c = getchar();
	int ret = 0, f = 1;
	while (c < '0' || c > '9') {
		f = c == '-'? -1: 1;
		c = getchar();
	}
	while ('0' <= c && c <= '9') {
		ret = ret * 10 + c - '0';
		c = getchar();
	}
	return ret * f;
}

inline int get_phi(int x) {
	int ret = x;
	for (int i = 2; i * i <= x; i++) {
		if (x % i == 0) {
			ret = ret / i * (i - 1);
			while (x % i == 0) {
				x /= i;
			}
		}
	}
	return x == 1? ret: ret / x * (x - 1);
}

inline int Pow(int w, int t, int MOD) {
	int ret = 1;
	for (; t; t >>= 1, w = (LL)w * w % MOD) {
		if (t & 1) {
			ret = (LL)ret * w % MOD;
		}
	}
	return ret;
}

inline int f(int p) {
	if (p == 1) {
		return 0;
	} else {
		int phi = get_phi(p);
		return Pow(2, f(phi) + phi , p);
	}
}

int main() {
	for (int i = read(); i; --i) {
		printf("%d\n", f(read())); 
	}
	return 0;
}

【日常小测】异或与区间加

相关链接

题目传送门:http://oi.cyo.ng/wp-content/uploads/2017/06/claris_contest_4_day2-statements.pdf
官方题解:http://oi.cyo.ng/wp-content/uploads/2017/06/claris_contest_4_day2-solutions.pdf

解题报告

这题又是一道多算法互补的题目
通过分类处理使复杂度达到$O((n+m)\sqrt{n})$
具体来讲是将以下两个算法结合:

1. 枚举右端点的值,若左端点的合法位置超过$\sqrt{n}$个

考虑每一个左右端点应该加减多少,使用前缀和技巧将复杂度优化到$O(n + m)$
具体细节不想写了,有点麻烦_(:з」∠)_
然后因为合法位置超过了$\sqrt{n}$个,所以这种情况至多出现$\sqrt{n}$个,复杂度符合要求

2. 其他情况

因为左端点不超过$\sqrt{n}$个,所以可以排序之后依次处理
使用分块来维护左端点的值,单次修改是$\sqrt{n}$的,单次查询是$O(1)$的

Code

#include<bits/stdc++.h>
#define LL long long
#define UI unsigned int
using namespace std;

const int N = 150009;
const int MOD = 1073741824;
const int blk_sz = 800;

int n, m, k, a[N];
UI a1[N], ans[N], blk_tag[N], tag[N];
vector<int> num, pos_list[N];
vector<pair<int, int> > left_list[N], right_list[N];
struct Query{
	int l, r, w;
	inline bool operator < (const Query &QQQ) const {
		return r > QQQ.r;
	} 
}q[N];

inline int read() {
	char c = getchar();
	int ret = 0, f = 1;
	while (c < '0' || c > '9') {
		f = c == '-'? -1: 1;
		c = getchar();
	}
	while ('0' <= c && c <= '9') {
		ret = ret * 10 + c - '0';
		c = getchar();
	}
	return ret * f;
}

inline int find(int x) {
	int l = 0, r = num.size() - 1, mid;
	while (l <= r) {
		mid = l + r >> 1;
		if (num[mid] == x) {
			return mid;
		} else if (num[mid] < x) {
			l = mid + 1;
		} else {
			r = mid - 1;
		}
	}
	return -1;
}

inline void solve(int A, int B) {
	static UI a2[N], cur;
	memset(a2, 0, sizeof(a2));
	for (int i = 1; i <= n; i++) {
		a2[i] = a2[i - 1] + (a[i] == num[B]);
	}
	cur = 0;
	for (int i = n; i; i--) {
		if (a[i] == num[B]) {
			cur += a1[i];
		}
		if (a[i - 1] == num[A]) {
			ans[i] += cur;
		}
		for (int j = 0; j < (int)left_list[i].size(); ++j) {
			cur -= (UI)left_list[i][j].second * (a2[left_list[i][j].first] - a2[i - 1]);
		}
	}
	memset(a2, 0, sizeof(a2));
	for (int i = 1; i <= n; ++i) {
		a2[i] = a2[i - 1] + (a[i - 1] == num[A]);
	}
	cur = 0;
	for (int i = 1; i <= n; i++) {
		if (a[i - 1] == num[A]) {
			cur -= a1[i];
		}
		if (a[i] == num[B]) {
			ans[i + 1] += cur;
		}
		for (int j = 0; j < (int)right_list[i].size(); ++j) {
			cur += (UI)right_list[i][j].second * (a2[i] - a2[right_list[i][j].first - 1]);
		}
	}
}

int main() {
	freopen("xor.in", "r", stdin);
	freopen("xor.out", "w", stdout);
	n = read(); m = read(); k = read();
	num.push_back(0);
	for (int i = 1; i <= n; ++i) {
		a[i] = a[i - 1] ^ read();
		num.push_back(a[i]);
	}
	sort(num.begin(), num.end());
	num.resize(unique(num.begin(), num.end()) - num.begin());
	for (int i = 0; i <= n; i++) {
		int pp = find(a[i]);
		pos_list[pp].push_back(i);
	}
	for (int i = 1, l, r, w; i <= m; ++i) {
		l = q[i].l = read();
		r = q[i].r = read();
		w = q[i].w = read();	
		left_list[l].push_back(make_pair(r, w));
		right_list[r].push_back(make_pair(l, w));
		a1[l] += w; 
		a1[r + 1] -= w;
	}
	sort(q + 1, q + 1 + m);
	for (int i = 1; i <= n; ++i) {
		a1[i] += a1[i - 1];
	}
	for (int i = 0; i < (int)num.size(); i++) {
		int r = i, l = find(num[i] ^ k);
		if (l != -1 && (int)pos_list[l].size() > blk_sz) {
			solve(l, r);
		}
	}
	for (int r = n, cur = 0; r; r--) {
		while (cur < m && q[cur + 1].r >= r) {
			++cur;
			for (int i = q[cur].l, lim = min(q[cur].r, (q[cur].l / blk_sz + 1) * blk_sz - 1); i <= lim; ++i) {
				tag[i] += q[cur].w;
			}
			for (int i = q[cur].l / blk_sz + 1, lim = q[cur].r / blk_sz - 1; i <= lim; ++i) {
				blk_tag[i] += q[cur].w;
			}
			for (int i = max(q[cur].r / blk_sz, q[cur].l / blk_sz + 1) * blk_sz; i <= q[cur].r; ++i) {
				tag[i] += q[cur].w;
			}
		}
		int t = find(a[r] ^ k);
		if (t != -1 && (int)pos_list[t].size() <= blk_sz) {
			for (int tt = 0; tt < (int)pos_list[t].size(); ++tt) {
				int l = pos_list[t][tt] + 1;
				if (l <= r) {
					ans[l] += tag[l] + blk_tag[l / blk_sz];
					ans[r + 1] -= tag[l] + blk_tag[l / blk_sz];
				} else {
					break;
				}
			}
		}
	}
	for (int i = 1; i <= n; i++) {
		ans[i] += ans[i - 1];
		printf("%d ", ans[i] % MOD);
	}
	return 0;
}

【日常小测】最长路径

相关链接

题目传送门:http://oi.cyo.ng/wp-content/uploads/2017/06/claris_contest_4_day2-statements.pdf
官方题解:http://oi.cyo.ng/wp-content/uploads/2017/06/claris_contest_4_day2-solutions.pdf

解题报告

首先我们要熟悉竞赛图的两个结论:

  1. 任意一个竞赛图一定存在哈密尔顿路径
  2. 一个竞赛图存在哈密尔顿回路当且仅当这个竞赛图强连通

现在考虑把强连通分量缩成一个点,并把新点按照哈密尔顿路径排列
那么$1$号点可以到达的点数就是$1$号点所在的$SCC$的点数加上$1$号点可以到达的其他$SCC$点数和

设$f_i$为$i$个点带标号竞赛图的个数
设$g_i$为$i$个点带标号强连通竞赛图的个数
有$f_i = 2^{\frac{n(n-1)}{2}}$
又有$g_i = f_i – \sum\limits_{j = 1}^{i – 1}{{{i}\choose{j}} g_j f_{i – j}}$

于是我们枚举$1$号点所在$SCC$的点数$i$和$1$号点可到达的其他$SCC$的点数和$j$
$ans_{x} = \sum\limits_{i = 1}^{x}{{{n – 1}\choose{i – 1}} g_i {{n – i}\choose{x – i}} f_{x – i} f_{n – x}}$

Code

#include<bits/stdc++.h>
#define LL long long
using namespace std;

const int N = 2009;

int n, MOD, f[N], g[N], pw[N * N], C[N][N];

inline int read() {
	char c = getchar();
	int ret = 0, f = 1;
	while (c < '0' || c > '9') {
		f = c == '-'? -1: 1;
		c = getchar();
	}
	while ('0' <= c && c <= '9') {
		ret = ret * 10 + c - '0';
		c = getchar();
	}
	return ret * f;
}

int main() {
	freopen("path.in", "r", stdin);
	freopen("path.out", "w", stdout);
	n = read(); MOD = read();
	pw[0] = 1;
	for (int i = 1; i < n * n; i++) {
		pw[i] = (pw[i - 1] << 1) % MOD;
	}
	C[0][0] = 1;
	for (int i = 1; i <= n; ++i) {
		C[i][0] = 1;
		for (int j = 1; j <= n; j++) {
			C[i][j] = (C[i - 1][j - 1] + C[i - 1][j]) % MOD;
		}
	}
	f[0] = g[0] = 1;
	for (int i = 1; i <= n; i++) {
		f[i] = g[i] = pw[i * (i - 1) >> 1];
		for (int j = 1; j < i; j++) {
			g[i] = (g[i] - (LL)C[i][j] * g[j] % MOD * f[i - j]) % MOD;
		}
	}
	for (int x = 1; x <= n; x++) {
		int ans = 0;
		for (int i = 1; i <= x; i++) {
			ans = (ans + (LL)C[n - 1][i - 1] * g[i] % MOD * C[n - i][x - i] % MOD * f[x - i] % MOD * f[n - x]) % MOD;
		}
		printf("%d\n", ans > 0? ans: ans + MOD);
	}
	return 0;
}

【算法笔记】Kosaraju算法

前言

今天考了一套$Claris$的题
$T1$就是$Claris$用$bitset$配合这个算法把求$SCC$优化到了$\frac{n^2}{32}$
吓得我赶紧来学了学

算法概述

$step \ 1.$ DFS一遍,记录下后序遍历的顺序
$step \ 2.$ 沿后续遍历的逆序、沿反向边再DFS一遍,每一个连通块即为一个SCC

算法解释

设原图为图$G$,将$SCC$缩点后的新图为$G’$
第一遍$DFS$保证了在若某个强连通分量$A$在$G’$中是另一个强连通分量$B$的祖先
那么在后续遍历的逆序中至少存在一个点$A$中的点在$B$中所有点的前面

这样在第二次$DFS$的时候,因为是逆序,所以只能往$G’$中的祖先走
又因为祖先的$SCC$已经标记过了,所以刚好把当前$SCC$搞完,不会有多

这算法太妙了

代码实现

int scc_cnt, vis[N];
vector<int> scc_cnt[N], que;
void DFS0(int w) {
	vis[w] = 0;
	for (int i = head[w]; i; i = nxt[i]) {
		if (!vis[to[i]]) {
			DFS0(to[i]);
		}
	}
	que[++tot] = w;
}
void DFS1(int w) {
	scc[scc_cnt].push_back(w);
	vis[w] = 1;
	for (int i = rev_head[w]; i; i = nxt[i]) {
		if (!vis[to[i]]) {
			DFS1(to[i]);
		}
	}
} 
void solve() {
	que.clear();
	memset(vis, 0, sizeof(vis));
	for (int i = 1; i <= n; i++) {
		if (!vis[i]) {
			DFS0(i);
		}
	}
	scc_cnt = 0;
	memset(vis, 0, sizeof(vis));
	for (int i = (int)que.size() - 1; ~i; i--) {
		if (!vis[que[i]]) {
			scc_cnt++;
			DFS1(que[i]);
		}
	}
}

使用$bitset$优化

这货复杂度是$O(m)$的
不难发现算法瓶颈在查找与某个点相邻的所有点中还没有被访问过的点
这个可以用$bitset$压位并配合__builtin开头的系列函数优化到$O(\frac{n^2}{32})$
例题的话,可以参考:友好城市

【日常小测】友好城市

相关链接

题目传送门:http://oi.cyo.ng/wp-content/uploads/2017/06/claris_contest_4_day2-statements.pdf
官方题解:http://oi.cyo.ng/wp-content/uploads/2017/06/claris_contest_4_day2-solutions.pdf

解题报告

这题的前置知识是把求$SCC$优化到$O(\frac{n^2}{32})$
具体来说,就是使用$bitset$配合$Kosaraju$算法

有了这个技能以后,我们配合$ST$表来实现提取一个区间的边的操作
这样的话,总的时间复杂度是:$O(\frac{(\sqrt{m} \log m + q) n^2}{32}+q \sqrt{m})$

然后我懒,没有用$ST$表,用的莫队,时间复杂度是$O(\frac{(m + q) n^2}{32}+q \sqrt{m})$
调一调块大小,勉勉强强卡过去了

Code

#include<bits/stdc++.h>
#define LL long long
#define UI unsigned int 
#define lowbit(x) ((x)&-(x))
using namespace std;

const int N = 159;
const int M = 300009;
const int QQ = 50009;
const int BlockSize = 1200;
const UI ALL = (1ll << 32) - 1;

int n, m, q, U[M], V[M], ans[QQ]; 
struct Query{
	int l, r, blk, id;
	inline bool operator < (const Query &Q) const {
		return blk < Q.blk || (blk == Q.blk && r < Q.r);
	}
}qy[QQ];
struct Bitset{
	UI v[5];
	inline void flip(int x) {
		v[x >> 5] ^= 1 << (x & 31);
	}
	inline void set(int x) {
		v[x >> 5] |= 1 << (x & 31);
	}
	inline void reset() {
		memset(v, 0, sizeof(v));
	}
	inline bool operator [](int x) {
		return v[x >> 5] & (1 << (x & 31));
	}
}g[N], rg[N], PreG[M / BlockSize + 9][N], PreRG[M / BlockSize + 9][N];

inline int read() {
	char c = getchar();
	int ret = 0, f = 1;
	while (c < '0' || c > '9') {
		f = c == '-'? -1: 1;
		c = getchar();
	}
	while ('0' <= c && c <= '9') {
		ret = ret * 10 + c - '0';
		c = getchar();
	}
	return ret * f;
}

inline void AddEdge(int u, int v, Bitset *a1, Bitset *a2) {
 	a1[u].set(v);
 	a2[v].set(u);
}

class Kosaraju{
	vector<int> que;
	Bitset vis;
public:
	inline int solve() {
		vis.reset();
		que.clear();
		for (int i = 1; i <= n; ++i) {
			if (!vis[i]) {
				dfs0(i);
			}
		}
		vis.reset();
		int ret = 0;
		for (int j = n - 1; ~j; j--) {
			int i = que[j];
			if (!vis[i]) {
				int cnt = dfs1(i);
				ret += cnt * (cnt - 1) / 2;
			}
		}
		return ret;
	}
private:
	inline void dfs0(int w) {
		vis.flip(w);
		for (int i = 0; i < 5; i++) {
			for (UI j = g[w].v[i] & (ALL ^ vis.v[i]); j; j ^= lowbit(j)) {
				int t = (__builtin_ffs(j) - 1) | (i << 5);
				if (!vis[t]) {
					dfs0(t);
				}
			}
		}
		que.push_back(w);
	}
	inline int dfs1(int w) {
		vis.flip(w);
		int ret = 1;
		for (int i = 0; i < 5; i++) {
			for (UI j = rg[w].v[i] & (ALL ^ vis.v[i]); j; j ^= lowbit(j)) {
				int t = (__builtin_ffs(j) - 1) | (i << 5);
				if (!vis[t]) {
					ret += dfs1(t);
				}
			}
		}
		return ret;
	}
}scc;

int main() {
	freopen("friend.in", "r", stdin);
	freopen("friend.out", "w", stdout);
	n = read(); m = read(); q = read();
	for (int i = 1; i <= m; i++) {
		U[i] = read();
		V[i] = read();
		AddEdge(U[i], V[i], PreG[i / BlockSize], PreRG[i / BlockSize]);
	}
	for (int i = 1; i <= q; i++) {
		qy[i].l = read(); 
		qy[i].r = read();
		qy[i].blk = qy[i].l / BlockSize;
		qy[i].id = i;
	}
	sort(qy + 1, qy + 1 + q);
	Bitset CurG[N], CurRG[N];
	for (int i = 1, L = 1, R = 0; i <= q; i++) {
		if (qy[i].blk != qy[i - 1].blk || i == 1) {
			L = qy[i].blk + 1;
			R = L - 1;	
			for (int j = 1; j <= n; j++) {
				CurG[j].reset();
				CurRG[j].reset();
			}
		}
		if (qy[i].r / BlockSize - 1 > R) {
			for (int j = R + 1, lim = qy[i].r / BlockSize - 1; j <= lim; j++) {
				for (int k = 1; k <= n; k++) {
					for (int h = 0; h < 5; h++) {
						CurG[k].v[h] ^= PreG[j][k].v[h];
						CurRG[k].v[h] ^= PreRG[j][k].v[h];
					}
				}
			}
			R = qy[i].r / BlockSize - 1;
		}
		if (L <= R) {
			for (int i = 1; i <= n; i++) {
				g[i] = CurG[i];
				rg[i] = CurRG[i];
			}
			for (int l = qy[i].l; l < L * BlockSize; l++) {
				AddEdge(U[l], V[l], g, rg);
			}
			for (int r = (R + 1) * BlockSize; r <= qy[i].r; r++) {
				AddEdge(U[r], V[r], g, rg);
			}
			ans[qy[i].id] = scc.solve();
		} else {
			for (int i = 1; i <= n; i++) {
				g[i].reset();
				rg[i].reset();
			}
			for (int j = qy[i].l; j <= qy[i].r; ++j) {
				AddEdge(U[j], V[j], g, rg);
			}
			ans[qy[i].id] = scc.solve();
		}
	}
	for (int i = 1; i <= q; i++) {
		printf("%d\n", ans[i]);
	}
	return 0;
}

【Codeforces 749E】Inversions After Shuffle

相关链接

题目传送门:http://codeforces.com/contest/749/problem/E
官方题解:http://codeforces.com/blog/entry/49186

解题报告

考虑从期望的定义下手
就是要求所有区间的逆序对的和
于是我们枚举右端点,然后算贡献
用$BIT$来维护,时间复杂度:$O(n \log n)$

Code

#include<bits/stdc++.h>
#define LL long long
#define lowbit(x) ((x)&-(x))
using namespace std;

const int N = 100009;

int n, p[N]; 

inline int read() {
	char c=getchar(); int ret=0,f=1;
	while (c<'0'||c>'9') {if(c=='-')f=-1;c=getchar();}
	while (c<='9'&&c>='0') {ret=ret*10+c-'0';c=getchar();}
	return ret*f;
}

class Fenwick_Tree{
	double sum[N];
	public:
		inline void init() {
			memset(sum, 0, sizeof(sum));
		}
		inline void modify(int p, int d = 1) {
			for (int i = p; i <= n; i += lowbit(i)) {
				sum[i] += d;
			}
		}
		inline double query(int p) {
			double ret = 0;
			for (int i = p; i; i -= lowbit(i)) {
				ret += sum[i];
			}
			return ret;
		}
}BIT;

int main() {
	n = read();
	for (int i = 1; i <= n; i++) {
		p[i] = read();
	}
	double ans = 0, psm = 0;
	for (int i = n; i; i--) {
		ans += BIT.query(p[i]);
		BIT.modify(p[i]);	
	}	
	ans *= n * (n + 1ll);
	BIT.init();
	for (int i = 1; i <= n; i++) {
		LL t1 = BIT.query(p[i]);
		LL t2 = i * (i - 1ll) / 2 - t1;
		ans += (psm += t1 - t2);
		BIT.modify(p[i], i);
	}
	printf("%.15lf\n", ans / n / (n + 1));
	return 0;
}

【BZOJ 4906】[BeiJing2017] 喷式水战改

相关链接

题目传送门:http://www.lydsy.com/JudgeOnline/problem.php?id=4906

解题报告

这题我们看一眼就知道可以用平衡树来维护序列
至于收益问题,我们发现区间信息可以合并
只需要记录一个区间左右两个端点的状态即可
于是搞一个$Splay$然后带$4^3$的常数来合并区间信息即可
总时间复杂度:$O(4^3n \log n)$

Code

这个代码win下没问题
ubuntu 16.04下也没有问题
UOJ的自定义测试也不会RE

然而一交到B站上就RE
我也很绝望啊,弃疗了_(:з」∠)_
反正北京省选测评也是在win下,就当a了吧

#include<bits/stdc++.h>
#define LL long long
using namespace std;

const int SGZ = 4;
const int N = 400000;

namespace Splay{
	int tot;
	struct Node *null, *root;
	struct Node{
		Node *ch[2], *fa;
		LL len, plen, adv[SGZ], sum[SGZ][SGZ];
		inline Node() {
		}
		inline Node(LL l, LL a, LL b, LL c) {
			memset(sum, 0, sizeof(sum));
			sum[0][0] = adv[0] = sum[3][3] = adv[3] = l * a;
			sum[1][1] = adv[1] = l * b;
			sum[2][2] = adv[2] = l * c;
			len = plen = l;
			ch[0] = ch[1] = fa = null;
			for (int i = 0; i < SGZ; i++) {
				for (int j = i; j < SGZ; j++) {
					for (int k = i - 1; ~k; k--) {
						sum[k][j] = max(sum[k][j], sum[i][j]);
					}
					for (int k = j + 1; k < SGZ; k++) {
						sum[i][k] = max(sum[i][k], sum[i][j]);
					}
				}
			}
		}
		inline void relax(LL &a, LL b) {
			a = b > a? b: a;
		}
		inline void maintain() {
			memset(sum, 0, sizeof(sum));
			//merge left son
			if (ch[0] != null) {
				for (int i = 0; i < SGZ; i++) {
					for (int k = SGZ - 1; k >= i; k--) {
						for (int j = k; j >= i; j--) {
							relax(sum[i][k], ch[0]->sum[i][j] + adv[k]);
						}
					}
				}
				plen = len + ch[0]->plen;
			} else {
				for (int i = 0; i < SGZ; i++) {
					sum[i][i] = adv[i];
				}
				plen = len;
			}
			//merge right son
			if (ch[1] != null) {
				for (int l = 0; l < SGZ; l++) {
					for (int i = SGZ - 1; i >= l; i--) {
						for (int r = SGZ - 1; r >= i; r--) {
							relax(sum[l][r], sum[l][i] + ch[1]->sum[i][r]);
						}
					}
				}
				plen += ch[1]->plen;
			} 
			for (int i = 0; i < SGZ; i++) {
				for (int j = i; j < SGZ; j++) {
					for (int k = i - 1; ~k; k--) {
						relax(sum[k][j], sum[i][j]);
					}
					for (int k = j + 1; k < SGZ; k++) {
						relax(sum[i][k], sum[i][j]);
					}
				}
			}
		}
		inline void rotate() {
			int t = fa->ch[1] == this;
			if (fa->fa != null) {
				int tt = fa->fa->ch[1] == fa;
				fa->fa->ch[tt] = this;
			}
			fa->ch[t] = ch[t^1]; ch[t^1]->fa = fa;
			ch[t^1] = fa; fa = ch[t^1]->fa;
			ch[t^1]->fa = this;
			ch[t^1]->maintain();
			maintain();
		}
		inline void splay(Node *ed) {
			while (fa != ed) {
				if (fa->fa != ed) {
					if ((fa->ch[0] == this) ^ (fa->fa->ch[0] == fa)) {
						rotate();
						rotate();
					} else {
						fa->rotate();
						rotate();
					}
				} else {
					rotate();
				} 
			}
 		}
 		inline LL ans() {
			LL ret = 0;
			for (int i = 0; i < SGZ; i++) {
				for (int j = i; j < SGZ; j++) {
					ret = max(ret, sum[i][j]);
				}
			} 
			return ret;
		}
		Node *find(LL pp) {
			if (ch[0]->plen >= pp) {
				return ch[0]->find(pp);
			} else if (ch[0]->plen + len >= pp) {
				return this;
			} else {
				return ch[1]->find(pp - ch[0]->plen - len);
			}
		}
		Node *min() {
			if (ch[0] != null) {
				return ch[0]->min();
			} else {
				return this;
			}
		}
	}p[N];
	inline void insert(LL pp, int a ,int b, int c, LL l) {
		p[++tot] = Node(l, a, b, c);
		Node *nw = p + tot;
		if (root != null) {
			Node *pos = root->find(pp);
			pos->splay(null); root = pos;
			int nlen = root->ch[0]->plen + root->len - pp, LEN = root->len, ls = root->ch[0] - p, rs = root->ch[1] - p;
			p[++tot] = Node(nlen, LEN? root->adv[0] / LEN: 0, LEN? root->adv[1] / LEN: 0, LEN? root->adv[2] / LEN: 0);
			*root = Node(LEN - nlen, LEN? root->adv[0] / LEN: 0, LEN? root->adv[1] / LEN: 0, LEN? root->adv[2] / LEN: 0);
			root->ch[0] = p + ls, root->ch[1] = p + rs;
			if (root->ch[1] != null) {
				root->ch[1]->min()->splay(root);
				p[tot].fa = root->ch[1];
				root->ch[1]->ch[0] = p + tot;
				nw->fa = p + tot;
				p[tot].ch[0] = nw;
				p[tot].maintain();
				root->ch[1]->maintain();
				root->maintain();
			} else {
				p[tot].fa = root;
				root->ch[1] = p + tot;
				nw->fa = p + tot;
				p[tot].ch[0] = nw;
				p[tot].maintain();
				root->maintain();
			}			
		} else {
			root = nw;
		}
	}
	inline LL query() {
		return root->ans();
	}
	inline void init() {
		null = root = p;
	}
};

int main() {
	Splay::init();
	int n; scanf("%d", &n);
	for (LL i = 1, LastAns = 0; i <= n; ++i) {
		LL pos, len, tmp, a, b, c;
		cin>>pos>>a>>b>>c>>len;
		Splay::insert(pos, a, b, c, len);
		tmp = Splay::query();
		printf("%lld\n", tmp - LastAns);
		LastAns = tmp;
	}
	return 0;
}

【Codeforces 814E】An unavoidable detour for home

相关链接

题目传送门:http://codeforces.com/contest/814/problem/E
官方题解:http://codeforces.com/blog/entry/52449
神犇题解Ⅰ:https://loj.ac/article/37
神犇题解Ⅱ:http://kczno1.blog.uoj.ac/blog/2660

解题报告

我们考虑分层图的话
那每一层的一个点一定会与上一层的某个点相连,剩下的度数只能层内消化
又因为这题对祖先有严格的限制,所以每一层都是原序列中连续的一段
于是$O(n^5)$的暴力$DP$就可以写了

至于$O(n^3)$的$DP$,就是$DP$套$DP$
预处理出转移的$buf$,然后转移的时候直接乘就好了
但这个预处理的时候要讨论很多情况啊,反正我自己考场上是想不出来的_(:з」∠)_

Code

#include<bits/stdc++.h>
#define LL long long
using namespace std;

const int N = 52;
const int MOD = 1000000007;
const int REV2 = 500000004;

int n, f[N][N], s2[N], s3[N], buf[N][N][N], C[N][N], prm[N]; 

inline int read() {
	char c=getchar(); int f=1,ret=0;
	while (c<'0'||c>'9') {if(c=='-')f=-1;c=getchar();}
	while (c<='9'&&c>='0') {ret=ret*10+c-'0';c=getchar();}
	return ret * f;
}

int main() {
	n = read();
	f[read() + 1][1] = 1;
	for (int i = 2; i <= n; i++) {
		s2[i] = s2[i - 1];
		s3[i] = s3[i - 1];
		if (read() == 2) {
			s2[i]++;
		} else {
			s3[i]++;
		}
	}
	C[0][0] = 1;
	for (int i = 1; i <= n; i++) {
		C[i][0] = 1;
		for (int j = 1; j <= i; j++) {
			C[i][j] = (C[i - 1][j - 1] + C[i - 1][j]) % MOD;
		}
	}
	prm[2] = 1;
	for (int i = 3; i <= n; i++) {
		prm[i] = REV2;
		for (int j = 2; j < i; j++) {
			prm[i] = (LL)prm[i] * j % MOD;
		}
	}
	for (int i = 0; i <= n; i++) {
		for (int j = 0; j <= n; j++) {
			for (int k = 0; k <= n; k++) {
				if (i == 0 && j == 0 && k == 0) {
					buf[i][j][k] = 1;
				} else if (i == 0 && j == 0) {
					for (int l = 2; l < k; l++) {
						(buf[i][j][k] += ((LL)buf[i][j][k - 1 - l] * C[k - 1][l] % MOD) * prm[l + 1] % MOD) %= MOD;
					}
				} else if (i == 0) {
					buf[i][j][k] = j > 1? (LL)(j - 1) * buf[i][j - 2][k] % MOD: 0;
					(buf[i][j][k] += k? (LL)k * buf[i][j][k - 1] % MOD: 0) %= MOD;
				} else {
					buf[i][j][k] = j? (LL)j * buf[i - 1][j - 1][k] % MOD: 0;
					(buf[i][j][k] += k? (LL)k * buf[i - 1][j + 1][k - 1] % MOD: 0) % MOD;
				} 
			}
		}
	} 
	for (int i = 1; i <= n; i++) {
		for (int j = 0; j < i; j++) {
			if (f[i][j]) {
				int t2 = s2[i] - s2[j];
				int t3 = s3[i] - s3[j];
				for (int k = i + 1; k <= n; k++) {
					f[k][i] = (f[k][i] + (LL)f[i][j] * buf[k - i][t2][t3]) % MOD;
				}
			}
		}
	} 
	int ans = 0;
	for (int i = 1; i <= n; i++) {
		ans = (ans + (LL)f[n][i] * buf[0][s2[n] - s2[i]][s3[n] - s3[i]]) % MOD;
	}
	cout<<ans<<endl;
	return 0;
}

【AtCoder】[Regular Contest 075 F] Mirrored

相关链接

题目传送门:http://arc075.contest.atcoder.jp/tasks/arc075_d

解题报告

之前PKUSC的时候考过$n + rev(n) = d$的版本
所以这一次把暴搜重新打一次就过了

Code

#include<bits/stdc++.h>
#define LL long long
using namespace std;

int *mth, *spj, a2[100], a1[100]; 

inline int read() {
	char c=getchar(); int f=1,ret=0;
	while (c<'0'||c>'9') {if(c=='-')f=-1;c=getchar();}
	while (c<='9'&&c>='0') {ret=ret*10+c-'0';c=getchar();}
	return ret * f;
}

LL DFS(LL res, LL l, LL r) {
	if (l <= r) {
		if (res == 0) {
			return l == r? 10: 1;
		} else {
			return 0;
		}
	} else {
		LL ret = 0, cur = l - r;
		l /= 10; r *= 10;
		for (int i = -9; i <= 9; i++) {
			if (abs(res - i * cur) < l * 11) {
				ret += mth[i] * DFS(res - i * cur, l, r);				
			}
		}
		return ret;
	}
}

int main() {
	mth = a1 + 50;
	spj = a2 + 50;
	for (int i = 0; i <= 9; i++) {
		for (int j = -9; j <= 0; j++) {
			mth[i + j]++;
		}
	}
	for (int i = 1; i <= 9; i++) {
		for (int j = -9; j <= 0; j++) {
			spj[i + j]++;
		}
	}
	LL ans = 0, D = -read();
	for (LL mx = 1e18; mx >= 10; mx /= 10) {
		LL delta = mx - 1;
		for (int i = -8; i <= 9; i++) {
			ans += spj[i] * DFS(D - i * delta, mx / 10, 10);
		}
	}
	cout<<ans<<endl;
	return 0;
}

【AtCoder】[Grand Contest 013 B] Hamiltonish Path

相关链接

题目传送门:http://agc013.contest.atcoder.jp/tasks/agc013_b

解题报告

最开始想用$DFS$树来搞
然而$Corner \ Case$太多了,写不动啊_(:з」∠)_

最后还是看了题解
题解是使用的调整算法
如果某个端点还可以走,那就走进去
因为每条边至多访问两遍,每个点至多访问一次
所以复杂度是:$O(n + m)$的

Code

#include<bits/stdc++.h>
#define LL long long
using namespace std;

const int N = 100009;
const int M = N << 1;

int n, m, head[N], nxt[M], to[M], vis[N];
deque<int> ans;

inline int read() {
	char c=getchar(); int f=1,ret=0;
	while (c<'0'||c>'9') {if(c=='-')f=-1;c=getchar();}
	while (c<='9'&&c>='0') {ret=ret*10+c-'0';c=getchar();}
	return ret * f;
}

inline void AddEdge(int u, int v) {
	static int E = 1;
	to[++E] = v; nxt[E] = head[u]; head[u] = E;
	to[++E] = u; nxt[E] = head[v]; head[v] = E; 
}

int main() {
#ifdef DBG
	freopen("11input.in", "r", stdin);
#endif	
	n = read(), m = read();
	for (int i = 1; i <= m; i++) {
		int u = read(), v = read();
		AddEdge(u, v);
		if (i == 1) {
			ans.push_back(u);
			ans.push_back(v);
			vis[u] = vis[v] = 1;
		}
	}
	for (int mk = 1; mk; ) {
		mk = 0;
		for (int i = head[ans.front()]; i; i = nxt[i]) {
			if (!vis[to[i]]) {
				mk = 1;
				ans.push_front(to[i]);
				vis[to[i]] = 1;
				break;
			}
		}
	}
	for (int mk = 1; mk; ) {
		mk = 0;
		for (int i = head[ans.back()]; i; i = nxt[i]) {
			if (!vis[to[i]]) {
				mk = 1;
				ans.push_back(to[i]);
				vis[to[i]] = 1;
				break;
			}
		}
	}
	cout<<ans.size()<<endl;
	for (; !ans.empty(); ans.pop_front()) {
		printf("%d ", ans.front());
	}
	return 0;
}