最大流+EK算法在于建图

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//建立一个超级源点和一个超级汇点，将cows与超级源点相连，容量为1//stalls与汇点连接，容量为1，然后cows与stalls连接，容量为1//求出最大流即可。//  main.cpp//  test//  Created by 吴有堃 on 2017/9/11//  Copyright © 2017年 吴有堃. All rights reserved#include <iostream>#include <stdio.h>#include <stdlib.h>#include <string>#include <cstring>#include <string.h>#include <math.h>#include <algorithm>#include <queue>#include <vector>#include <map>#define LL long long#define inf 0x3f3f3f3f#define mod 1e9+7using namespace std;const int maxn=505;LL cap[maxn][maxn],flow[maxn][maxn],rest[maxn];int snode,tnode,pre[maxn],vis[maxn];bool BFS_Path(int m)//augmenting path{    memset(vis, 0, sizeof(vis));    queue<int>Q;    int u=0,v=0;    u=snode; pre[u]=u;    rest[u]=inf;vis[u]=1;    Q.push(u);    while (!Q.empty()) {        u=Q.front(); Q.pop();        for(v=0;v<=m;v++){            if(!vis[v]&&flow[u][v]<cap[u][v]){                pre[v]=u;vis[v]=1;                rest[v]=min(rest[u],cap[u][v]-flow[u][v]);                if(v==tnode) return true; // find augment path                Q.push(v);            }        }    }    return false;//no augment path}LL max_flow(int s, int t, int m){    snode=s;tnode=t;    LL ans=0;    while (BFS_Path(m)) {        ans+=rest[t];        int v=t;        while (v!=s) {            int u=pre[v];            flow[u][v]+=rest[t];            flow[v][u]-=rest[t];            v=u;        }    }    return ans;}int main(){    int N=0,M=0;    while (scanf("%d %d",&N,&M)!=EOF) {        memset(cap, 0, sizeof(cap));        memset(flow, 0, sizeof(flow));        memset(pre, 0, sizeof(pre));        memset(rest, 0, sizeof(rest));        int s=0,t=N+M+1,num=0,a=0;        for(int i=1;i<=N;i++){            scanf("%d",&num);            cap[s][i]=1; //源点s与每头牛链接；            while (num--) {                scanf("%d",&a);                cap[i][N+a]=1;//每头牛和对应的stall链接            }        }        for(int i=1;i<=M;i++){//每个stall与汇点t相联            cap[i+N][t]=1;        }        cout<<max_flow(s,t,t)<<endl;    }    return 0;}