ZOJ Problem Set

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

Circle

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Time Limit: 1 Second      Memory Limit: 32768 KB

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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 3
1 2
2 3
1 3

4 4
1 2
2 3
3 1
1 4

Sample Output

YES
NO




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;
}