Task — Largest multiple

You are given $N, a list of positive single digit integers (0-9). Write a script to find the largest multiple of 2 that can be formed from the list.

Examples

Example 1
Input: @N = (1, 0, 2, 6)
Output: 6210

Example 2
Input: @N = (1, 4, 2, 8)
Output: 8412

Example 3
Input: @N = (4, 1, 7, 6)
Output: 7614

Analysis

The answer to this is:

• Reverse sort the list
• Find the last occurrence of an even digit $s, which will be the smallest even digit in the list
• Concatenate
  1. all the digits preceding the last occurrence of $s
  2. all the digits following that $s
  3. $s

Conveniently that can be done with a single regex.

Try it 

#!/usr/bin/perl

# Blog: http://ccgi.campbellsmiths.force9.co.uk/challenge

use v5.26;    # The Weekly Challenge - 2021-05-31
use utf8;     # Week 115 - task 2 - Largest multiple
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

largest_multiple(1, 2, 3, 4, 5);
largest_multiple(1, 0, 2, 6);
largest_multiple(1, 4, 2, 8);
largest_multiple(4, 1, 7, 6);
largest_multiple(1, 3, 5, 7);
largest_multiple(4, 8, 6, 2);

my @x;
push @x, int(rand(10)) for 1 .. 20;
largest_multiple(@x);

sub largest_multiple {
    
    my ($numbers);
    
    # initialise
    say qq[\nInput:  (] . join(', ', @_) . ')';
    
    # concatenate digits in descending order
    $numbers = join('', reverse sort {$a <=> $b} @_);
    
    # find smallest multiple of 2 and move it to the end
    if ($numbers =~ m|^([0-9]*)([02468])([0-9]*)|) {
        say qq[Output: $1$3$2];
    } else {        
        say qq[Output: no multiple of 2 in input];
    }
}

last updated 2026-03-29 — 8 lines of code

Output

Input:  (1, 2, 3, 4, 5)
Output: 54312

Input:  (1, 0, 2, 6)
Output: 6210

Input:  (1, 4, 2, 8)
Output: 8412

Input:  (4, 1, 7, 6)
Output: 7614

Input:  (1, 3, 5, 7)
Output: no multiple of 2 in input

Input:  (4, 8, 6, 2)
Output: 8642

Input:  (7, 9, 0, 3, 4, 9, 7, 4, 6, 7, 8, 1, 5, 0, 7, 1,
   3, 8, 8, 6)
Output: 99888777766544331100