FZU - 2085 幸运图

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int MAXN = 1005;

int n,m;
int G[MAXN][MAXN];

int main(){
    while (scanf("%d%d",&n,&m) != EOF){
        int a,b,flag = 0;
        memset(G,0,sizeof(G));
        for (int i = 0; i < m; i++){
           scanf("%d%d",&a,&b);
           if (a == b)
               flag = 1;
            G[a][b]++,G[b][a]++;
            if (G[a][b] >= 2 || G[b][a] >= 2)
                flag = 1;
        }
        if (flag)
            printf("No\n");
        else printf("Yes\n");
    }
    return 0;
}