tarjan做法：

#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#define LL long long
#define uLL unsigned long long
using namespace std;
const int N = 1e5 + 5;
const int INF = 0x3f3f3f3f;
int T, m, n, pos, sum, id, cnt, tot, top, st[N], num[N], col[N], vis[N], low[N], dfn[N], to[N], ne[N], head[N], a[N], b[N];
bool flag;
void add(int x, int y){
    to[++id] = y, ne[id] = head[x], head[x] = id;
}
void init(){
	pos = sum = id = cnt = tot = top = 0;
	memset (st, 0, sizeof(st));
	memset (num, 0, sizeof(num));
	memset (col, 0, sizeof(col));
	memset (vis, 0, sizeof(vis));
	memset (dfn, 0, sizeof(dfn));
	memset (to, 0, sizeof(to));
	memset (ne, 0, sizeof(ne));
	memset (head, 0, sizeof(head));
	memset (a, 0, sizeof(a));
	memset (b, 0, sizeof(b));
}
void tarjan(int u, int no_x, int no_y){
    st[++top] = u, vis[u] = 1, dfn[u] = low[u] = ++cnt;
    for (int i = head[u]; i; i = ne[i]){
        int v = to[i];
        if (v == no_y && no_x == u)
        	continue;
        if (!dfn[v]){
            tarjan(v, no_x, no_y);
            low[u] = min(low[u], low[v]);
        }
        else if (vis[v])
            low[u] = min (low[u], dfn[v]);
    }
    if (dfn[u] == low[u]){
        tot++;
        while (1){
            vis[ st[top] ] = 0, num[tot]++, col[ st[top] ] = tot, top--;
            if (st[top + 1] == u)
                break;
        }
    }
}
int main(){
	cin >> T;
	while (T--){
		init();
		cin >> n >> m;
		flag = 1;
		for (int i = 1; i <= m; i++){
		    cin >> a[i] >> b[i];
		    add(a[i], b[i]);
		}
		for (int j = 1; j <= m; j++){
			for (int i = 1; i <= n; i++)
			    if (!dfn[i])
			        tarjan(i, a[j], b[j]);
		}
		for (int i = 1; i <= m; i++)
		    if (col[ a[i] ] != col[ b[i] ]){
		    	cout << "YES\n";
		    	flag = 0;
		    	break;
		    }
		if (flag)
			cout << "NO\n"; 
	}
    return 0;
}