hdu 3342 Legal or Not

  1/* ***********************************************
  2Author :111qqz
  3Created Time :2015年12月17日 星期四 19时29分00秒
  4File Name :code/hdoj/3342.cpp
  5************************************************ */
  6
  7#include <cstdio>
  8#include <cstring>
  9#include <iostream>
 10#include <algorithm>
 11#include <vector>
 12#include <queue>
 13#include <set>
 14#include <map>
 15#include <string>
 16#include <cmath>
 17#include <cstdlib>
 18#include <ctime>
 19#define fst first
 20#define sec second
 21#define lson l,m,rt<<1
 22#define rson m+1,r,rt<<1|1
 23#define ms(a,x) memset(a,x,sizeof(a))
 24typedef long long LL;
 25#define pi pair < int ,int >
 26#define MP make_pair
 27
 28using namespace std;
 29const double eps = 1E-8;
 30const int dx4[4]={1,0,0,-1};
 31const int dy4[4]={0,-1,1,0};
 32const int inf = 0x3f3f3f3f;
 33const int N=1E2+7;
 34
 35int n,m;
 36
 37bool v[N][N];
 38int in[N];
 39
 40void topo()
 41{
 42    queue<int>q;
 43
 44    for ( int i = 0 ; i < n ; i++)
 45    {
 46	if (in[i]==0) q.push(i);
 47    }
 48
 49    int cnt = 0 ;
 50    while (!q.empty())
 51    {
 52	cnt++;
 53	int u = q.front(); q.pop();
 54
 55	for ( int i = 0 ; i< n ; i++)
 56	{
 57	    if (!v[u][i]) continue;
 58
 59	    in[i]--;
 60	    if (in[i]==0)
 61	    {
 62		q.push(i);
 63	    }
 64	}
 65    }
 66    if (cnt==n)
 67    {
 68	puts("YES");
 69    }
 70    else
 71    {
 72	puts("NO");
 73    }
 74}
 75int main()
 76{
 77	#ifndef  ONLINE_JUDGE 
 78	freopen("code/in.txt","r",stdin);
 79  #endif
 80
 81	while (~scanf("%d %d",&n,&m)!=EOF)
 82	{
 83	    if (n==0) break;
 84	    ms(v,false);
 85	    ms(in,0);
 86	    while (m--)
 87	    {
 88		int x,y;
 89		scanf("%d %d",&x,&y);
 90		if (v[x][y]) continue ; //重边?
 91		v[x][y] = true;
 92		in[y]++;
 93	    }
 94
 95	    topo();
 96	}
 97
 98  #ifndef ONLINE_JUDGE  
 99  fclose(stdin);
100  #endif
101    return 0;
102}