UVA 10510 Cactus

#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<string>
#include<string.h>
#include<math.h>
#include<limits.h>
#include<time.h>
#include<stdlib.h>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<vector>
#define LL long long
using namespace std;
int n, m;
vector <int> edge[20005];
int DFN[20005], Low[20005];
int cnt = 1;
int ans = 0;
int fa[20005], out[20005];
int flag = 1;
void sign(int v, int u)
{
    while(fa[u] != v)
    {
        out[u]++;
        if(out[u] > 1)
        {
            flag = 0;
            return ;
        }
        u = fa[u];
    }
}
void tarjan(int v)
{
    DFN[v] = Low[v] = cnt++;
    int len = edge[v].size();
    for(int i = 0; i < len; i++)
    {
        int u = edge[v][i];
        if(!DFN[u])
        {
            fa[u] = v;
            tarjan(u);
            Low[v] = min(Low[v], Low[u]);
        }
        else
        {
            Low[v] = min(Low[v], DFN[u]);
            sign(u, v);
            if(!flag)
                return ;
        }
    }
    if(DFN[v] == Low[v])
    {
        int tmp;
        ans++;
        if(ans > 1)
        {
            flag = 0;
            return ;
        }
    }
}
int main()
{
    int T;
    scanf("%d", &T);
    while(T--)
    {
        cnt = 1;
        ans = 0;
        flag = 1;
        scanf("%d%d", &n, &m);
        for(int i = 0; i < n; i++)
            edge[i].clear();
        for(int i = 0; i < m; i++)
        {
            int v, u;
            scanf("%d%d", &v, &u);
            edge[v].push_back(u);
        }
        memset(DFN, 0, sizeof DFN);
        memset(fa, 0, sizeof fa);
        memset(out, 0, sizeof out);
        for(int i = 0; i < n; i++)
            if(!DFN[i])
                tarjan(i);
        if(flag)
            printf("YES\n");
        else
            printf("NO\n");
    }
    return 0;
}