AtCoder Beginner Contest 392 E

https://atcoder.jp/contests/abc392/tasks/abc392_e

Union-Findを使ってグラフを連結していきますが、連結に寄与しない、すなわちrootが同じになるエッジをためておいてあとで連結に使います。

// Cables and Servers
#![allow(non_snake_case)]

use std::collections::HashSet;


//////////////////// library ////////////////////

fn read<T: std::str::FromStr>() -> T {
    let mut line = String::new();
    std::io::stdin().read_line(&mut line).ok();
    line.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>().split_whitespace()
            .map(|e| e.parse().ok().unwrap()).collect()
}


//////////////////// Graph ////////////////////

type Node = usize;
type Edge = (Node, Node);

fn read_edge() -> Edge {
    let v: Vec<Node> = read_vec();
    let (A, B) = (v[0] - 1, v[1] - 1);
    (A, B)
}


//////////////////// UnionFind ////////////////////

use std::cmp::max;

struct UnionFind {
    parents: Vec<Node>,
    heights: Vec<i32>,
}

impl UnionFind {
    fn new(N: usize) -> UnionFind {
        let parents: Vec<Node> = (0..N).collect();
        let heights: Vec<i32> = vec![1; N];
        UnionFind { parents, heights }
    }
    
    fn is_root(&self, v: Node) -> bool {
        self.parents[v] == v
    }
    
    fn connect(&mut self, u: Node, v: Node) {
        let r1 = self.root(u);
        let r2 = self.root(v);
        if r1 == r2 {
            return
        }
        
        let h1 = self.heights[r1];
        let h2 = self.heights[r2];
        if h1 <= h2 {   // r2にr1がぶら下がる
            self.parents[r1] = r2;
            self.heights[r2] = max(self.heights[r2], self.heights[r1]+1);
        }
        else {
            self.parents[r2] = r1;
            self.heights[r1] = max(self.heights[r1], self.heights[r2]+1);
        }
    }
    
    fn root(&self, v0: Node) -> Node {
        if self.is_root(v0) {
            v0
        }
        else {
            let p = self.parents[v0];
            self.root(p)
        }
    }
}


//////////////////// process ////////////////////

fn read_input() -> (usize, Vec<Edge>) {
    let v: Vec<usize> = read_vec();
    let (N, M) = (v[0], v[1]);
    let edges: Vec<Edge> = (0..M).map(|_| read_edge()).collect();
    (N, edges)
}

fn F(N: usize, edges: Vec<Edge>) {
    let mut tree = UnionFind::new(N);
    let mut extra_edges: Vec<(usize, Edge)> = vec![];
    for (i, edge) in edges.into_iter().enumerate() {
        let (A, B) = edge;
        let r1 = tree.root(A);
        let r2 = tree.root(B);
        if r1 == r2 {
            extra_edges.push((i, edge))
        }
        else {
            tree.connect(r1, r2)
        }
    }
    
    let mut set_roots: HashSet<Node> = (0..N).filter(|&v| tree.is_root(v)).
                                                                    collect();
    let L = set_roots.len();
    println!("{}", L-1);
    for &(k, (A, _)) in &extra_edges[..L-1] {
        let r1 = tree.root(A);
        let &r2 = set_roots.iter().filter(|&&r| r != r1).next().unwrap();
        tree.connect(r1, r2);
        if tree.root(r2) == r2 {
            set_roots.remove(&r1);
        }
        else {
            set_roots.remove(&r2);
        }
        println!("{} {} {}", k+1, A+1, r2+1)
    }
}

fn main() {
    let (N, edges) = read_input();
    F(N, edges)
}