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 Rust (1.15.1) (judge verdict: Accepted)

fn main() {
  let (n, m): (usize, usize) = read_tuple();
  let edges: Vec<(usize, usize)> = read_tuples(m);
  
  let mut ans = 0;
  for (v,w) in edges.clone() {
    let removed = edges.iter().filter(|&&e|e!=(v,w)).collect::<Vec<_>>();
    let mut uf = UnionFind::new(n);
    for &(v,w) in removed {
      uf.unite(v-1, w-1);
    }
    if uf.count() != 1 {
      ans += 1;
    }
  }
  
  println!("{:?}", ans);
}

struct UnionFind {
  n: usize,
  rp: Vec<usize>,
  rk: Vec<usize>
}

impl UnionFind {
  fn new(n: usize) -> UnionFind {
    UnionFind {
      n: n,
      rp: (0..n).collect(),
      rk: vec![1; n]
    }
  }

  fn rep(&self, p: usize) -> usize {
    if self.rp[p] == p {p} else {self.rep(self.rp[p])}
  }

  fn rank(&self, p: usize) -> usize {
    self.rk[self.rep(p)]
  }

  fn count(&self) -> usize {
    let mut reps = (0..self.n).map(|x|self.rep(x)).collect::<Vec<_>>();
    reps.dedup();
    reps.len()
  }
  
  fn unite(&mut self, p: usize, q:usize) {
    let rkp = self.rank(p);
    let rkq = self.rank(q);
    let (p, q) = if rkp <= rkq {(p, q)} else {(q, p)};

    let repp = self.rep(p);
    let repq = self.rep(q);
    self.rp[p] = repq;
    self.rp[repp] = repq;
    self.rk[repq] += self.rk[repp];
  }
}       

fn read_tuple<T1: std::str::FromStr, T2: std::str::FromStr>() -> (T1, T2) {
  let mut buf = String::new();
  std::io::stdin().read_line(&mut buf).ok();
  let mut it = buf.trim().split_whitespace();
  let x = it.next().unwrap().parse::<T1>().ok().unwrap();
  let y = it.next().unwrap().parse::<T2>().ok().unwrap();
  (x, y)
}

fn read_tuples<T1: std::str::FromStr, T2: std::str::FromStr>(n: usize) -> Vec<(T1, T2)> {
  let mut vec: Vec<(T1, T2)> = vec![];
  for _ in 0 .. n {
    vec.push(read_tuple());
  }
  vec
}