Problem:
Score : 300 points

Problem Statement

You are given an undirected connected graph with N vertices and M edges that does not contain self-loops and double edges.

The i-th edge (1 \leq i \leq M) connects Vertex a_i and Vertex b_i.

An edge whose removal disconnects the graph is called a bridge.

Find the number of the edges that are bridges among the M edges.

Notes

A self-loop is an edge i such that a_i=b_i (1 \leq i \leq M).

Double edges are a pair of edges i,j such that a_i=a_j and b_i=b_j (1 \leq i<j \leq M).

An undirected graph is said to be connected when there exists a path between every pair of vertices.

Constraints

2 \leq N \leq 50

N-1 \leq M \leq min(N(N−1)⁄2,50)

1 \leq a_i<b_i \leq N

The given graph does not contain self-loops and double edges.

The given graph is connected.

Input

Input is given from Standard Input in the following format:

N M
a_1 b_1
a_2 b_2
:
a_M b_M

Output

Print the number of the edges that are bridges among the M edges.

Sample Input 1

7 7
1 3
2 7
3 4
4 5
4 6
5 6
6 7

Sample Output 1

4

The figure below shows the given graph:

The edges shown in red are bridges. There are four of them.

Sample Input 2

3 3
1 2
1 3
2 3

Sample Output 2

0

It is possible that there is no bridge.

Sample Input 3

6 5
1 2
2 3
3 4
4 5
5 6

Sample Output 3

5

It is possible that every edge is a bridge.

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.Scanner;

public class Main {

	static int n;
	static int m;
	static int a[], b[];
	static boolean visited[];
	static boolean graph[][];

	static void DFS(int v) {
		visited[v] = true;
		for (int v2 = 0; v2 < n; v2++) {
			if (graph[v][v2] == false)
				continue;
			if (visited[v2] == true)
				continue;
			DFS(v2);
		}
		return;
	}

	public static void main(String[] args) {
		int count = 0;
		try (Scanner sc = new Scanner(System.in)) {
			n = sc.nextInt();
			m = sc.nextInt();
			visited = new boolean[n];
			graph = new boolean[n][n];
			a = new int[m];
			b = new int[m];

			for (int i = 0; i < n; i++) {
				for (int j = 0; j < n; j++) {
					graph[i][j] = false;
				}
			}

			for (int i = 0; i < m; i++) {
				a[i] = sc.nextInt() - 1;
				b[i] = sc.nextInt() - 1;
				graph[a[i]][b[i]] = graph[b[i]][a[i]] = true;
			}
			sc.close();
		}
		
		for (int i = 0; i < m; i++) {
			graph[a[i]][b[i]] = graph[b[i]][a[i]] = false;
			for (int j = 0; j < n; j++) {
				visited[j] = false;
			}
			DFS(0);
			boolean eval = true;
			for (int j = 0; j < n; j++) {
				if (visited[j] == false) {
					eval = false;
					break;
				}
			}
			if (!eval) {
				count++;
			}
			graph[a[i]][b[i]] = graph[b[i]][a[i]] = true;
		}
		System.out.println(count);
	}
}