二分图最大匹配 。

用临街矩阵些超时，改用邻接表，结果对vector忘清零，错了好几次。

http://acm.nyist.net/JudgeOnline/problem.php?pid=239

#include<vector>#include<stdio.h>#include<string.h>using namespace std;#define maxn 502vector<int>v[maxn];int use[maxn],path[maxn];bool dfs(int x){    for(int i=0;i<v[x].size();i++)    {        int e=v[x][i];        if(!use[e])        {            use[e]=1;            if(!path[e]||dfs(path[e]))             {                 path[e]=x;                return true ;             }        }    }    return false ;}int match(int n){    int sum=0;    memset(path,0,sizeof(path));    for(int i=1;i<=n;i++)    {        memset(use,0,sizeof(use));        if(dfs(i)) sum++;    }    return sum;}int main(){    int i,n,m,ncase,a,b;    scanf("%d",&ncase);    while(ncase--)    {        scanf("%d%d",&n,&m);        memset(v,0,sizeof(v));        for(i=1;i<=m;i++)        {            scanf("%d%d",&a,&b);            v[a].push_back(b);        }        printf("%d\n",match(n));    }    return 0;}