645D - Robot Rapping Results Report - CodeForces Solution

#include <bits/stdc++.h>
using namespace std;
#define IOS ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
const int N=1e5+5,mod=1e9+7;
typedef long long ll;
vector<int> v[N];
int d[N],n,m,ans[N],cnt,a[N],b[N],s;
bool topsort()
{
	queue<int> q;
	for(int i=1;i<=n;i++) if(!d[i]) q.push(i);
	while(q.size())
	{
		if(q.size()>1) return false;
		int x=q.front();
		q.pop();
		for(int i=0;i<v[x].size();i++)
		{
			int j=v[x][i];
			if(--d[j]==0)
			{
				q.push(j);
				ans[j]=x;
				s++;
			}
		}
	}
	return true;
}
void solve()
{
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++)
	{
		scanf("%d%d",&a[i],&b[i]);
		v[a[i]].push_back(b[i]);
		d[b[i]]++;
	}
	if(!topsort()) printf("-1\n");
	else
	{
		for(int i=1;i<=m;i++)
		{
			if(ans[b[i]]==a[i])
			{
				s--;
				if(!s) 
				{
					printf("%d\n",i);
					return;
				}
			}
		}
	}
}
int main()
{
	solve();
	return 0;
}