SPOJ1436(Is it a tree)

1436. Is it a tree Problem code: PT07Y

You are given an unweighted, undirected graph. Write a program to check if it's a tree topology.

Input

The first line of the input file contains two integers N and M --- number of nodes and number of edges in the graph (0 < N <= 10000, 0 <= M <= 20000). Next M lines contain M edges of that graph --- Each line contains a pair (u, v) means there is an edge between node u and node v (1 <= u,v <= N).

Output

Print YES if the given graph is a tree, otherwise print NO.

Example

Input:
3 2
1 2
2 3
Output:
YES

//2009-05-17 12:22:31     Xredman    Is it a tree    accepted     0.10      2.9M      C++ 
#include <iostream>
#include <vector>
using namespace std;

const int N = 10005;

bool Visited[N];
vector<int> V[N];
int n, m;
int cnt;

void init()
{
    for(int i = 1; i <= n; i++)
        V[i].clear();
}


void dfs(int cc)
{
    vector<int>::iterator p;
    p = V[cc].begin();
    
    while(p != V[cc].end())
    {
        if(!Visited[*p])
        {
            Visited[*p] = true;
            cnt++;
            dfs(*p);
        }
        p++;
    }
}

int main()
{
    int u, v, k;
    while(scanf("%d%d", &n, &m) != EOF)
    {
        

        init();
        for(k = 0; k < m; k++)
        {
            scanf("%d%d", &u, &v);

            V[u].push_back(v);
            V[v].push_back(u);
        }
        if(n == 1)
        {
            printf("YES\n");
            continue;
        }
        if(m != n -1)
        {
            printf("NO\n");
            continue;
        }
        memset(Visited, false, sizeof(Visited));
        Visited[1] = true;
        cnt = 1;
        dfs(1);

        if(cnt == n)
            printf("YES\n");
        else
            printf("NO\n");
    }
    return 0;
}

转载于:https://www.cnblogs.com/Xredman/archive/2009/05/17/1458896.html