4.7 网易C++笔试

消息封城问题 莫名一直10% 求大佬指教

#include<iostream>
#include<stdio.h>
#include<set>
using namespace std;
int n,m,s;
set<int> to;
int x_[100010],y_[100010];
int main(){
    cin>>n>>m>>s;
    for(int i=1;i<=m;++i){
        int x,y;
        cin>>x_[i]>>y_[i];
        
    }
    for(int i=1;i<=m;++i){
        int x,y;
        x=x_[i];
        y=y_[i];
        if(x==0){
            if(y==s){
                cout<<i;
                return 0;
            }else{
                to.insert(y);                        
            }
        }else{
            if(x==s){
                if(y!=s) 
                    to.insert(y);
                
            }else if(y==s){
                if(x!=s)
                    to.insert(x);
            }
        }
        if(to.size()==n-1){
            cout<<m;
            return 0;
        }
    }
    cout<<0;
    return 0;
}