Task — Last member

You are given an array of positive integers, @ints. Pick the biggest and next biggest members (x, y), apply the following rules, and repeat until you are left with 1 member (report it) or none (report 0).

Rules:
a) if x == y then remove both members
b) if x != y then remove both members and add new member y - x

Analysis

The rules are quite straightforward. Clearly the order of the integers is not material, so to make things easy I keep them in reverse sorted order.

That means that x and y are always the first two elements and I can quickly remove them using shift, and then if they differ, add the difference to @ints using unshift.

And I do all that as long as at least 2 elements remain, and when they don't, we have the answer.

Try it 

#!/usr/bin/perl

use v5.16;    # The Weekly Challenge - 2023-06-19
use utf8;     # Week 222 task 2 - Last member
use strict;   # Peter Campbell Smith
use warnings; # Blog: http://ccgi.campbellsmiths.force9.co.uk/challenge

last_member(2, 7, 4, 1, 8, 1);
last_member(1);
last_member(1, 1);

# longer example
my ($j, @ints);
for $j (0 .. 9) {
    $ints[$j] = int(rand(49)) + 1;
}
last_member(@ints);

sub last_member {
    
    my (@ints, $diff, $legend);
    
    # initialise
    @ints = @_;
    $legend = '';
    
    # loop until 0 or 1 left
    while (scalar @ints > 1) {
        
        # sort remaining list in decreasing order
        @ints = sort { $b <=> $a } @ints;
        
        # remove two largest, and add difference if they differ
        $legend .= qq[\n  Pick $ints[0] and $ints[1], we remove both ];
        $diff = (shift @ints) - (shift @ints);
        unshift @ints, $diff if $diff;
        
        # and say what we've done
        $legend .= qq[and add new member $diff ] if $diff;
        $legend .= scalar @ints > 0 ? qq[=> (] . join(', ', @ints) . q[)] : q[and are left with none];
    }
    
    # report findings
    say qq[\nInput: \@ints = (] . join(', ', @_) . q[)];
    say qq[Output: ] . (defined $ints[0] ? $ints[0] : '0') . $legend;
}

Output

Input: @ints = (2, 7, 4, 1, 8, 1)
Output: 1
  Pick 8 and 7, we remove both and add new member 1 => (1, 4, 2, 1, 1)
  Pick 4 and 2, we remove both and add new member 2 => (2, 1, 1, 1)
  Pick 2 and 1, we remove both and add new member 1 => (1, 1, 1)
  Pick 1 and 1, we remove both => (1)

Input: @ints = (1)
Output: 1

Input: @ints = (1, 1)
Output: 0
  Pick 1 and 1, we remove both and are left with none

Input: @ints = (4, 8, 20, 41, 27, 43, 41, 9, 8, 11)
Output: 2
  Pick 43 and 41, we remove both and add new member 2 => (2, 41, 27, 20, 11, 9, 8, 8, 4)
  Pick 41 and 27, we remove both and add new member 14 => (14, 20, 11, 9, 8, 8, 4, 2)
  Pick 20 and 14, we remove both and add new member 6 => (6, 11, 9, 8, 8, 4, 2)
  Pick 11 and 9, we remove both and add new member 2 => (2, 8, 8, 6, 4, 2)
  Pick 8 and 8, we remove both => (6, 4, 2, 2)
  Pick 6 and 4, we remove both and add new member 2 => (2, 2, 2)
  Pick 2 and 2, we remove both => (2)