http://acm.timus.ru/problem.aspx?space=1&num=1280
      开始做这题，感觉巨简单，然后直接Topological Sorting，然后随便整，结果各种WR，后来细想一下，应该在建图后，每读入一个点，就判断当前点是否入度为零，不是的话，直接NO，否则删掉当前点的出边，继续读点处理，直到处理完N个点，就OK啦！

Code：
#include<cstdio>

struct Edg{
   int n;
   int next;
}EE[100001];

int head[1011];
int rd[1011];
int top;
int n;
int m;

int Topological_Sorting(int u){
   int i = 0;
   int reVal = 0;
   if(!rd[u]){
      int q = head[u];
      while(q != -1){
         rd[EE[q].n]--;
         q = EE[q].next;
      }
      reVal = 1;
   }
   return reVal;
}

int main(int argc, char* argv[], char* env[])
{
   int i = 0;
   int j = 0;
   int I = 0;
   int J = 0;
   int isCorrect = 0;
   while(scanf("%d%d", &n, &m) != EOF){
      for(i = 1; i <= n; i++){
         head[i] = -1;
         rd[i] = 0;
      }
      top = 0;
      while(m-- > 0){
         scanf("%d %d", &I, &J);
         rd[J]++;
         EE[top].n = J;
         EE[top].next = head[I];
         head[I] = top++;
      }
      isCorrect = 1;
      for(i = 1; i <= n; i++){
         scanf("%d", &I);
         if(isCorrect){
            isCorrect = Topological_Sorting(I);
         }
      }
      printf("%s\n", isCorrect == 1 ? "YES" : "NO");
   }
   return 0;
}