1707.cpp

#include <stdio.h>
#include <iostream>
#include <vector>
#include <memory.h>
using namespace std;
int t, v, e; int color[20001]; bool check;
vector<int> adj[20001]; 
void search(int now, int col) {
	//visited[now] = true;
	color[now] = col;
	for (int i = 0; i < adj[now].size(); i++) {
		int next = adj[now][i];
		if (color[next] == 0) {
			if (col == 1) search(next, 2);
			else if (col == 2) search(next, 1);
		}
	}
}
int main()
{

	scanf("%d", &t);
	while (t--) {
		scanf("%d %d", &v, &e);
		check = true;
		for (int i = 1; i <= v; i++) adj[i].clear();
		memset(color, 0, sizeof(int) * 20001);

		for (int i = 0; i < e; i++) {
			int a, b; scanf("%d %d", &a, &b);
			adj[a].push_back(b);
			adj[b].push_back(a);
		}

		for (int i = 1; i <= v; i++) {
			if (color[i] == 0) search(i, 1);
		}

		for (int i = 1; i <= v; i++) {
			for (int j = 0; j < adj[i].size(); j++) {
				int next = adj[i][j];
				if (color[i] == color[next]) {
					check = false;
					break;
				}
			}
		}

		if (check) printf("YES\n");
		else printf("NO\n");

	}

    return 0;
}