Circle Detect

: Circle Detect

时间限制:10000ms

单点时限:1000ms

内存限制:256MB

描述

You are given a directed graph G which has N nodes and M directed edges. Your task is to detect whether it contains any circle.  

输入

The first line contains an integer T denoting the number of test cases. (1 <= T <= 5)  

For each test case the first line contains two integers N and M. (1 <= N, M <= 100000)  

Then follows M lines. Each contains two integers u and v denoting there is an edge from u to v. (1 <= u, v <= N)

输出

For each test case output "YES" or "NO" denoting whether there is a circle in the graph.

样例输入

2
5 5  
1 2  
2 3  
4 5  
5 4  
4 2
3 2
1 2
2 3

样例输出

YES  
NO 

判断是否有环

用dfs或bfs

import java.util.*;
public class Main {
	static ArrayList<Integer>[]  g = new ArrayList[100005];
	static int[] vis = new int[100005];
	static boolean dfs(int u)
	{
		vis[u] = 1;
		for(int i = 0; i < g[u].size(); i++){
			int v = g[u].get(i);
			if (vis[v] == 1){
				return true;
			}
			if (vis[v] == 0){
				if (dfs(v)) {
					return true;
				}
			}
		}
		vis[u] = 2;
		return false;
	}
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

		for(int i = 0; i < 100005; i++){
			g[i] = new ArrayList();
		}
		int t = sc.nextInt();

		for(int i = 0; i < t; i++){
			int n = sc.nextInt();
			for(int j = 1; j <= n; j++){
				g[j].clear();
			}
			Arrays.fill(vis, 0);

			int m = sc.nextInt();
			for(int j = 0; j < m; j++){
				int x = sc.nextInt();
				int y = sc.nextInt();
				g[x].add(y);
			}
			int found = 0;
			for(int j = 1; j <= n; j++){
				if (vis[j] == 0 && dfs(j)){
					found = 1;
					break;
				}
			}
			if (found == 0){
				System.out.println("NO");
			} else {
				System.out.println("YES");
			}
		}
		
	}	
}