相关链接
题目传送门：http://www.lydsy.com/JudgeOnline/problem.php?id=4735
神犇题解：http://blog.csdn.net/neither_nor/article/details/64919143
吐槽
我只会$O(mn2^n)$的暴力$DP$
然而正解长成这样，为什么$n$给这么小？
给人一种这题有妙妙的做法的错觉……
解题报告
我们把正常的牌看成$-1$的话，那么实际上就是要让每一个地方的前缀和$\ge 1$
现在考虑在末尾加上一个$-1$，那么是不是就是处末尾以外的地方$\ge 1$，而末尾的前缀和为$0$？
现在考虑把这$m+1$个元素拿出来做一个环排列
仔细观察可以发现：一种环排列能且仅能对应一种合法摆放
于是我们求出环排列的个数就可以了，最后再去一下多加的那个$-1$的影响
最终答案：$\frac{m!}{m-n+1}$
Code
#include<bits/stdc++.h>
#define LL long long
using namespace std;
const int MOD = 998244353;
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() {
int n = read(), m = 0, ans = 1;
for (int i=1;i<=n;i++) m += read();
for (int i=2;i<=m;i++) ans = ((LL)ans * ((i!=m-n+1)?i :1)) % MOD;
cout<<ans;
return 0;
}
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