题目链接：http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemCode=3321

Your task is so easy. I will give you an undirected graph, and you just need to tell me whether the graph is just a circle. A cycle is three or more nodes V₁, V₂, V₃, ... V_k, such that there are edges between V₁ and V₂, V₂ and V₃, ... V_k and V₁, with no other extra edges. The graph will not contain self-loop. Furthermore, there is at most one edge between two nodes.

Input

There are multiple cases (no more than 10).

The first line contains two integers n and m, which indicate the number of nodes and the number of edges (1 < n < 10, 1 <= m < 20).

Following are m lines, each contains two integers x and y (1 <= x, y <= n, x != y), which means there is an edge between node x and node y.

There is a blank line between cases.

Output

If the graph is just a circle, output "YES", otherwise output "NO".

Sample Input

3 31 22 31 34 41 22 33 11 4

Sample Output

YESNO




改了队友的经典并查集代码过了此题，记录下作为经典模板使用

#include<iostream>using namespace std;int pre[1010];int find(int x){    return pre[x]==x?x:pre[x]=find(pre[x]);}int main(){    int road,root1,root2,num,i,start,end,total;    while(cin>>num>>road )    {        int sum=0;        total=num-1;        for(i=1; i<=num; i++)            pre[i]=i;        while(road--)        {            cin>>start>>end;            sum+=(start+end);            root1=find(start);            root2=find(end);            if(root1!=root2)            {                pre[root1]=root2;                total--;            }        }        int tmp=0;        for(i=1;i<=num;i++)        {            tmp+=i;        }        if(!total && tmp*2 == sum)        {            cout<<"YES"<<endl;        }        else        {            cout<<"NO"<<endl;        }    }    return 0;}