hdoj-3342-Legal or Not（拓扑排序）

题目链接

 /*
     Name:hdoj-3342-Legal or Not
     Copyright:
     Author:
     Date: 2018/4/11 15:59:18
     Description:
         判断是否存在环
 */
 #include <iostream>
 #include <queue>
 #include <vector>
 #include <cstring>
 #include <algorithm>
 using namespace std;
 const int MAXN = 1e+;
 int du[MAXN], n , m, L[MAXN];
 vector<int>  g[MAXN];
 bool topsort() {
     memset(du, , sizeof(du));
     for (int i=; i<n; i++) {
         for (int j=; j<g[i].size(); j++) {
             du[g[i][j]]++;
         }
     }
     int tot = ;
     queue<int> Q;
     for (int i=; i<n; i++) {
         if (!du[i]) {
             Q.push(i);
         }
     }
     while (!Q.empty()) {
         int x = Q.front();
         Q.pop();
         L[tot++] = x;
         for (int j=; j<g[x].size(); j++) {
             int t = g[x][j];
             du[t]--;
             if (!du[t]) {
                 Q.push(t);
             }
         }
     }
     if (tot == n) return ;
     return ;
 }
 int main()
 {
     while (cin>>n>>m && (m || n)) {
         memset(L, , sizeof(L));
         memset(g, , sizeof(g));
         for (int i=; i<m; i++) {
             int a, b;
             cin>>a>>b;
             g[a].push_back(b);
         }
         if (topsort() == ) cout<<"YES"<<endl;
         else cout<<"NO"<<endl;
     }
     return ;
 }