Kuhn's algorithm

import core.bitop;
import std.algorithm;
import std.array;
import std.container;
import std.conv;
import std.exception;
import std.functional;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.typecons;
import std.file;
import std.bigint;

string TASK = "pairs";

alias pair = Tuple !(int, "l", int, "r");

int[][] e;
int[] pr, used;
pair[] ans;

int timer = 1;

bool dfs(int st){
    used[st] = timer;
    foreach (to; e[st]){
        if (pr[to] == -1 || (used[pr[to]] < timer && dfs(pr[to]))){
            pr[to] = st;
            return true;
        }
    }
    return false;
}

void main(){
    File fin = File(TASK ~ ".in", "r");
    File fout = File(TASK ~ ".out", "w");
    int n, m;
    fin.readf("%s %s", &n, &m);
    fin.readln;
    e = new int[][](n, 0);
    foreach (i; 0..n){
        e[i] = fin.readln.split.map !(to !(int)).array;
        e[i] = e[i][0..e[i].length - 1];
        e[i][] -= 1;
    }
    pr = new int[m];
    used = new int[n];
    pr[] -= 1;
    foreach (v; 0..n){
        if (dfs(v))
            ++timer;
    }
    ans = new pair[0];
    foreach (i; 0..m){
        if (pr[i] != -1){
            ans ~= pair(pr[i] + 1, i + 1);
        }
    }
    fout.writeln(ans.length);
    foreach (a; ans){
        fout.writeln(a.l, " ", a.r);
    }
}