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.*;
class Main {
    static int N,M;
    static int[] visited, x,y,edges;
    static int[][] graph;
    static int ans;
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        N = sc.nextInt();
        M = sc.nextInt();
        visited = new int[N+1];
        edges = new int[M];
        graph = new int[N+1][N+1];
        for(int i = 1; i<N+1; i++) visited[i] = -1;
        x = new int[M];
        y = new int[M];
        for(int i = 0; i<M; i++) {
            x[i] = sc.nextInt();
            y[i] = sc.nextInt();
            graph[x[i]][y[i]] = graph[y[i]][x[i]] = 1;
        }

        for(int i = 0; i<M; i++) {
            for(int j = 1; j<N+1; j++) visited[j] = -1;
            graph[x[i]][y[i]] = graph[y[i]][x[i]] = 0;
            dfs(1);
//            System.out.println();
//            for(int v: visited) System.out.print(v);
//            System.out.println();
            for(int j = 0; j<N+1; j++) {
                if(visited[j] == 1) continue;
                if(visited[j] == -1) {
                    ans++;
                    break;
                }
            }
            graph[x[i]][y[i]] = graph[y[i]][x[i]] = 1;
        }
        System.out.println(ans);
    }
    public static void dfs(int node) {
        if(visited[node]==1) return;
        //System.out.print(node+" ");
        if(visited[node] == -1) {
            visited[node] = 1;
        }
        for(int i = 0; i<N+1; i++) {
            if(graph[node][i] == 1) dfs(i);
        }
    }
}