题解 | 连通图

#include<iostream>
#include<vector>
using namespace std;
int main(){
    int n,m;
    while(cin>>n>>m){
        if(n==0&&m==0){
            break;
        }
        else{
            vector<int>bianji(n+1,-1);
            int count=0;
            for(int i=0;i<m;i++){
                int x,y;
                cin>>x>>y;
                int a=x,b=y;
                while(bianji[a]!=-1)    a=bianji[a];
                while(bianji[b]!=-1)    b=bianji[b];
                if(a==b)    continue;
                else{
                    bianji[b]=x;
                }
            }
            for(int i=1;i<=n+1;i++){
                if(bianji[i]==-1){
                    count++;
                }
            }
            if(count==1){
                cout<<"YES"<<endl;
            }
            else    cout<<"NO"<<endl;
        }
    }
}