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.*;

public class Main {
	static ArrayList<Integer>[] lists;

	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		lists = new ArrayList[n + 1];
		for (int i = 1; i <= n; i++) {
			lists[i] = new ArrayList<Integer>();
		}
		int[] a = new int[m];
		int[] b = new int[m];
		for (int i = 0; i < m; i++) {
			a[i] = sc.nextInt();
			b[i] = sc.nextInt();
			lists[a[i]].add(b[i]);
			lists[b[i]].add(a[i]);
		}
		int count = 0;
		for (int i = 0; i < m; i++) {
			boolean[] test = new boolean[n + 1];
			Arrays.fill(test, false);
			search(1, a[i], b[i], test);
			if (!isAllTrue(test)) {
				count++;
			}
		}
		System.out.print(count);
	}
	
	static void search(int idx, int a, int b, boolean[] test) {
		for (int i : lists[idx]) {
			if (!test[i] && !((a == i && b == idx) || (a == idx && b == i))) {
				test[i] = true;
				search(i, a, b, test);
			}
		}
	}
	
	static boolean isAllTrue(boolean[] bs) {
		for (int i = 1; i < bs.length; i++) {
			if (!bs[i]) {
				return false;
			}
		}
		return true;
	}
}