CodeForces - 645D Robot Rapping Results Report

Posted 2020-12-03

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#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=1e5+5;

int hd[N],to[N],ne[N],num,n,m;
int d[N],ans=-1,mid;

inline void add(int x,int y){ to[++num]=y,ne[num]=hd[x],hd[x]=num;}

inline bool check(){
	memset(d,0,sizeof(d)); int x=0,y;
	for(int i=1;i<=mid;i++) d[to[i]]++;
	for(int i=1;i<=n;i++) if(!d[i]) if(!x) x=i; else return 0;

	for(;x;){
		y=x,x=0;
		for(int i=hd[y];i;i=ne[i]) if(i<=mid&&!(--d[to[i]]))
			if(!x) x=to[i]; else return 0;
	}

    return 1;
}

int main(){
	scanf("%d%d",&n,&m);
	for(int i=1,uu,vv;i<=m;i++)
		scanf("%d%d",&uu,&vv),add(uu,vv);

	int l=1,r=m;
	while(l<=r){
		mid=l+r>>1;
		if(check()) ans=mid,r=mid-1;
		else l=mid+1;
	}

	printf("%d
",ans);
	return 0;
}