注意点： 输入数据中可能有重复，需要进行处理！
1 #include <stdio.h>
2 #include <iostream>
3 #include <cstring>
4 #include <vector>
5 #include <algorithm>
6 #include <sstream>
7
8
9 using namespace std;
10
11 int n, m;
12
13 int indegree[101];
14 int g[101][101];
15
16 int main()
17 {
18 while(scanf("%d %d", &n, &m) != EOF)
19 {
20 if(n == 0 && m == 0)
21 break;
22
23 memset(indegree, 0, sizeof(indegree));
24 memset(g, 0, sizeof(g));
25
26 int a, b;
27 for(int i = 1; i <= m; ++i)
28 {
29 scanf("%d %d", &a, &b);
30
31 if(g[a][b] == 0) // 输入中有可能有反复输入如1,2；1,2这种情况！！！
32 {
33 g[a][b] = 1;
34 indegree[b]++;
35 }
36
37 }
38
39 int cnt = 0;
40
41 for(int k = 0; k < n; ++k)
42 {
43 int i, j;
44 for(i = 0; i < n; ++i)
45 {
46 if(indegree[i] == 0)
47 {
48 indegree[i] = -1;
49 cnt++;
50 break;
51 }
52 }
53
54 for(j = 0; j < n; ++j)
55 {
56 if(g[i][j] == 1)
57 {
58 indegree[j]--;
59 g[i][j] == 0;
60 }
61 }
62 }
63
64
65 if(cnt == n)
66 cout << "YES" << endl;
67 else
68 cout << "NO" << endl;
69 }
70
71 return 0;
72 }
