【HDOJ】5154 Harry and Magical Computer

拓扑排序。

 /* 5154 */
 #include <iostream>
 #include <cstdio>
 #include <cstring>
 #include <cstdlib>
 #include <queue>
 #include <vector>
 using namespace std;

 #define MAXN 105

 vector<int> vec[MAXN];
 vector<int>::iterator iter;
 int cnt[MAXN];
 int n, m;

 void init() {
     int i;

     memset(cnt, , sizeof(cnt));
     for (i=; i<=n; ++i)
         vec[i].clear();
 }

 bool topSort() {
     int i, j, k, tmp;
     int c = ;
     queue<int> Q;

     for (i=; i<=n; ++i) {
         if (cnt[i] == ) {
             Q.push(i);
         }
     }

     while (!Q.empty()) {
         ++c;
         i = Q.front();
         Q.pop();
         for (iter=vec[i].begin(); iter!=vec[i].end(); ++iter) {
             if (--cnt[*iter] == ) {
                 Q.push(*iter);
             }
         }
     }

     if (c < n)
         return false;
     else
         return true;
 }

 int main() {
     int i, j, k;
     bool flag;

     #ifndef ONLINE_JUDGE
         freopen("data.in", "r", stdin);
     #endif

     while (scanf("%d %d", &n, &m) != EOF) {
         init();
         for (i=; i<m; ++i) {
             scanf("%d %d", &j, &k);
             vec[j].push_back(k);
             ++cnt[k];
         }
         flag = topSort();
         if (flag)
             printf("YES\n");
         else
             printf("NO\n");
     }

     return ;
 }