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 Kotlin (1.0.0) (judge verdict: Accepted)

import java.util.*

fun main(args: Array<String>) {
    val sc = Scanner(System.`in`)
    val n = sc.nextInt()
    val m = sc.nextInt()
    val ab = (0 until m).map { sc.next().toInt() to sc.next().toInt() }
    println(problem075c(n, m, ab))
}

fun problem075c(n: Int, m: Int, ab: List<Pair<Int, Int>>): Int {
    var count = 0
    for (i in 0 until m) {
        val routes = Array(n) { mutableListOf<Int>() }
        for (j in 0 until m) {
            if (i == j) continue
            val (a, b) = ab[j]
            routes[a - 1].add(b - 1)
            routes[b - 1].add(a - 1)
        }
        if (routes.find { it.isEmpty() } != null) {
            count++
            continue
        }

        val visited = Array(n) { false }
        val queue = ArrayDeque<Int>()
        queue.offer(routes[0][0])
        visited[routes[0][0]] = true
        while (!queue.isEmpty()) {
            val current = queue.poll()
            for (k in 0 until routes[current].size) {
                val next = routes[current][k]
                if (visited[next]) continue
                visited[next] = true
                queue.offer(next)
            }
        }
        if (visited.count { !it } != 0) count++
    }
    return count
}