C++ 代码
#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 100010;
int res = 0;
int a[N],b[N],t[N];
int main()
{
int n,k;
cin>>n>>k;
for(int i = 1;i <= n; i++ ) t[i] = 0;
t[0] = 1;
for(int i = 1;i <= k; i ++)
{
cin>>a[i]>>b[i];
if(t[b[i]] == 0) {
res ++;
//cout<<i<<endl;
}
t[a[i]] ++;
}
cout<<res<<endl;
}