Description
N (1 ≤ N ≤ 100) cows, conveniently numbered 1..N, are participating in a programming contest. As we all know, some cows code better than others. Each cow has a certain constant skill rating that is unique among the competitors.
The contest is conducted in several head-to-head rounds, each between two cows. If cow A has a greater skill level than cow B (1 ≤ A ≤ N; 1 ≤ B ≤ N; A ≠ B), then cow A will always beat cow B.
Farmer John is trying to rank the cows by skill level. Given a list the results of M (1 ≤ M ≤ 4,500) two-cow rounds, determine the number of cows whose ranks can be precisely determined from the results. It is guaranteed that the results of the rounds will not be contradictory.
Input
* Line 1: Two space-separated integers: N and M
* Lines 2..M+1: Each line contains two space-separated integers that describe the competitors and results (the first integer, A, is the winner) of a single round of competition: A and B
Output
* Line 1: A single integer representing the number of cows whose ranks can be determined
Sample Input
5 5
4 3
4 2
3 2
1 2
2 5
分析：有人说叫闭包传递。这题就是用一种类似于floyd的算法，
开始时，如果a胜b则由a到b连一条边。这样所有a能走到的点都是排名在a以后的。
所有能走到a的点都是排名在a以前的。用floyd，求出每个点能到达的点。
如果有一个点，排名在它之前的和排名在它之后的点之和为n-1，那么它的排名就是确定的。
#include<iostream>
using namespace std ;
int main()
{
int N,M,a,b;
cin>>N>>M;
int aa[110][110]={0};
while(M--)
{
cin>>a>>b;
aa[a][b]=1;
} for(int i=1;i<=N;i++)
for(int j=1;j<=N;j++)
for(int k=1;k<=N;k++)
{
if(aa[j][i]&&aa[i][k])
aa[j][k]=1;
}
int ans=0;
for(int i=1;i<=N;i++)
{
int tmp=0;
for(int j=1;j<=N;j++)
tmp+=aa[i][j]+aa[j][i];
if(tmp==N-1)ans++; } cout<<ans<<endl;
return 0;
}