题目链接：
HDU 2444 The Accomodation of Students
分析：
用BFS判断二分图，用匈牙利算法求最大匹配。
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
#include <cmath>
#include <string>
#include <queue>
using namespace std;
const int MAX_N=210;
const int MAX_M=40010;int n,m,total;
int head[MAX_N],vis[MAX_N],match[MAX_N],color[MAX_N];struct Edge{int to,next;Edge() {}Edge(int to,int next) : to(to),next(next) {}
}edge[MAX_M];inline void AddEdge(int from,int to)
{edge[total].to=to;edge[total].next=head[from];head[from]=total++;
}inline bool IsBipartiteGraph()
{int st=1;memset(color,-1,sizeof(color));while(1){int find=0;for(int i=st;i<=n;i++){if(color[i]==-1){st=i;find=1;break;}}if(find==0) break;queue<int> que;color[st]=1;//color[i]=0和color[j]=0是不同的两组que.push(st);st++;while(!que.empty()){int cur=que.front();que.pop();for(int i=head[cur];i!=-1;i=edge[i].next){int v=edge[i].to;if(color[v]==-1){color[v]=1-color[cur];que.push(v);}else if(color[v]==color[cur]){return false;}}}}return true;
}inline bool dfs(int u)
{for(int i=head[u];i!=-1;i=edge[i].next){int to=edge[i].to;if(vis[to]==0){vis[to]=1;if(match[to]==-1 || dfs(match[to])){match[to]=u;match[u]=to;return true;}}}return false;
}inline void solve()
{int ans=0;memset(match,-1,sizeof(match));for(int i=1;i<=n;i++){if(match[i]==-1){memset(vis,0,sizeof(vis));if(dfs(i)) ans++;}}printf("%d\n",ans);
}int main()
{while(~scanf("%d%d",&n,&m)){total=0;memset(head,-1,sizeof(head));for(int i=0;i<m;i++){int from,to;scanf("%d%d",&from,&to);AddEdge(from,to);}if(IsBipartiteGraph()==false){printf("No\n");continue;}solve();}return 0;
}