hdu_5154 拓扑

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<vector>
#include<queue>
using namespace std;
int n,m;
const int maxn=110;
vector<int> g[maxn];
int du[maxn];
bool topsort()
{
    memset(du,0,sizeof(du));
    for( int i=1;i<=n;i++){
        for( int j=0;j<g[i].size();j++)
            du[g[i][j]]++;
    }
    int tot=0;
    queue<int>q;
    for(int i=1;i<=n;i++)
        if(!du[i]) q.push(i);
       // printf("%d",q.front());
    while(!q.empty()){
        int x=q.front(); q.pop();
        tot++;
        for(unsigned int j=0;j<g[x].size();j++){
            int t = g[x][j];
            du[t]--;
            if(!du[t])
                q.push(t);
        }
    }
    if(tot==n) return 1;
    return 0;
}
int main()
{
    while(scanf("%d%d",&n,&m)!=EOF){
        int x,y;
        int state=1;
        for(int i=0;i<maxn;i++)
        g[i].clear();
        for(int i=0;i<m;i++){
            scanf("%d%d",&x,&y);
            g[x].push_back(y);
        }
        state = topsort();
        if(state==1) printf("YES
");
        else printf("NO
");
    }
    return 0;
}