CodeForces 645D Robot Rapping Results Report

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<iostream>
using namespace std;
typedef long long LL;
const double pi=acos(-1.0),eps=1e-6;
void File()
{
    freopen("D:\\in.txt","r",stdin);
    freopen("D:\\out.txt","w",stdout);
}
template <class T>
inline void read(T &x)
{
    char c=getchar(); x=0;
    while(!isdigit(c)) c=getchar();
    while(isdigit(c)) {x=x*10+c-‘0‘; c=getchar();}
}

const int maxn=100010;
int n,m,u[maxn],v[maxn];
int h[maxn],sz,r[maxn];
struct Edge{int u,v,nx;}e[maxn];

void add(int a,int b)
{
    e[sz].u=a; e[sz].v=b; e[sz].nx=h[a];
    h[a]=sz++;
}

bool check(int x)
{
    memset(h,-1,sizeof h); sz=0;
    memset(r,0,sizeof r);
    for(int i=1;i<=x;i++) add(u[i],v[i]),r[v[i]]++;
    queue<int>Q;

    int num=0; for(int i=1;i<=n;i++) if(r[i]==0) Q.push(i),num++;
    
    if(num!=1) return 0;

    while(!Q.empty())
    {
        int top=Q.front(); Q.pop();
        num=0;

        for(int i=h[top];i!=-1;i=e[i].nx)
        {
            r[e[i].v]--;
            if(r[e[i].v]==0)
            {
                num++;
                Q.push(e[i].v);
            }
        }

        if(num>1) return 0;
    }
    return 1;
}

int main()
{
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++) scanf("%d%d",&u[i],&v[i]);
    check(4);

    int L=1,R=m,ans=-1;
    while(L<=R)
    {
        int m=(L+R)/2;
        if(check(m)) ans=m,R=m-1;
        else L=m+1;
    }
    printf("%d\n",ans);
    return 0;
}