题目链接
1 /*
2 Name:hdoj-3342-Legal or Not
3 Copyright:
4 Author:
5 Date: 2018/4/11 15:59:18
6 Description:
7 判断是否存在环
8 */
9 #include <iostream>
10 #include <queue>
11 #include <vector>
12 #include <cstring>
13 #include <algorithm>
14 using namespace std;
15 const int MAXN = 1e+5;
16 int du[MAXN], n , m, L[MAXN];
17 vector<int> g[MAXN];
18 bool topsort() {
19 memset(du, 0, sizeof(du));
20 for (int i=0; i<n; i++) {
21 for (int j=0; j<g[i].size(); j++) {
22 du[g[i][j]]++;
23 }
24 }
25 int tot = 0;
26 queue<int> Q;
27 for (int i=0; i<n; i++) {
28 if (!du[i]) {
29 Q.push(i);
30 }
31 }
32 while (!Q.empty()) {
33 int x = Q.front();
34 Q.pop();
35 L[tot++] = x;
36 for (int j=0; j<g[x].size(); j++) {
37 int t = g[x][j];
38 du[t]--;
39 if (!du[t]) {
40 Q.push(t);
41 }
42 }
43 }
44 if (tot == n) return 1;
45 return 0;
46 }
47 int main()
48 {
49 while (cin>>n>>m && (m || n)) {
50 memset(L, 0, sizeof(L));
51 memset(g, 0, sizeof(g));
52 for (int i=0; i<m; i++) {
53 int a, b;
54 cin>>a>>b;
55 g[a].push_back(b);
56 }
57 if (topsort() == 1) cout<<"YES"<<endl;
58 else cout<<"NO"<<endl;
59 }
60 return 0;
61 }
