题目传送门:http://cogs.top/cogs/problem/problem.php?pid=14
离线版题目:http://paste.ubuntu.com/18772859/
二分图匹配板题,Dinic跑一跑就ok啦!
#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
inline int read(){
char c=getchar(); int buf=0,f=1;
while (c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}
while (c<='9'&&c>='0'){buf=buf*10+c-'0';c=getchar();}
return buf*f;
}
const int MAXN = 10000+9;
const int INF = 1000000000;
int n,m,a,b,T,head[MAXN],to[MAXN],nxt[MAXN],cap[MAXN],vout;
int s,t,cur[MAXN],que[MAXN],fro,bak,dis[MAXN],flow[MAXN];
inline void AddEdge(int a, int b){
to[++T] = b; nxt[T] = head[a]; head[a] = T; cap[T] = 1;
to[++T] = a; nxt[T] = head[b]; head[b] = T;
}
inline bool BFS(){
memset(dis,-1,sizeof(dis));
dis[s] = 0; que[fro=bak=1]=s;
while (bak <= fro){
int w = que[bak++];
for (int i=head[w];i;i=nxt[i])
if (flow[i] < cap[i] && dis[to[i]]==-1)
dis[to[i]] = dis[w] + 1,
que[++fro] = to[i];
}
if (dis[t] != -1) return true;
else return false;
}
int MaxFlow(int w, int f){
if (w == t) return f;
else {
int ret = 0;
for (int &i=cur[w];i;i=nxt[i]){
if (flow[i] < cap[i] && dis[to[i]] == dis[w]+1){
int tmp = MaxFlow(to[i], min(f, cap[i]-flow[i]));
flow[i] += tmp; flow[i^1] -= tmp;
ret += tmp; f-=tmp;
if (!f) return ret;
}
}
return ret;
}
}
inline void Dinic(){
while (BFS()){
for (int i=0;i<=n*2+1;i++)
cur[i] = head[i];
vout += MaxFlow(s,INF);
}
}
int main(){
freopen("flyer.in","r",stdin);
freopen("flyer.out","w",stdout);
n = read(); m = read();
s = 0; t = 2*n+1; T = 1;
while (~scanf("%d%d",&a,&b)) AddEdge(a*2,b*2-1);
for (int i=1;i<=m;i++) AddEdge(s,i*2-1);
for (int i=m+1;i<=n;i++) AddEdge(i*2,t);
for (int i=1;i<=n;i++) AddEdge(i*2-1,i*2);
Dinic();
printf("%d\n",vout);
return 0;
}