Here we are with TASK #1 from The Weekly Challenge #130. Enjoy!

The Challenge

You are given an array of positive integers, such that all the numbers appear even number of times except one number.

Write a script to find that integer.

Example 1

Input: @N = (2, 5, 4, 4, 5, 5, 2)
Output: 5 as it appears 3 times in the array where as all other numbers 2 and 4 appears exactly twice.

Example 2

Input: @N = (1, 2, 3, 4, 3, 2, 1, 4, 4)
Output: 4

The Questions

From an interview’s point of view, I think that one question might be how many items we expect to receive as inputs. It is explicitly stated that they come from an array, so it should be something manageable in memory and should not need to resort to any fancy way of dealing with a huge amount of input data. The solution, anyway, assumes that it’s OK to track the different input numbers with a hash of flags, which might grow as much as about one half of the number of element of the input array.

The Solution

We will try to solve a slightly wider problem, i.e. finding all elements that appear an odd number of times inside the input array.

To do this, we use a hash to keep track all items that appear an odd number of times. This is initially empty, because by default all numbers appear 0 times, i.e. an even number of times. We then start iterating over the provided array:

  • if the element is in the hash, it means that so far we spotted it an odd number of times. With this particular instance, it makes an even number of times, so we remove the item from the hash;
  • if the item is not present, it either never appeared, or it was removed in a previous iteration, so we put it in the hash because this occurrence is an odd one.

When we go past the last element, the hash contains keys for all items that appear an odd number of times. If the input is compliant with the requirements… it will also be the only one.

This is the implementation in Raku:

#!/usr/bin/env raku

sub odd-number (*@inputs) {
   my %is-odd;
   for @inputs -> $element {
      if %is-odd{$element} { %is-odd{$element}:delete }
      else                 { %is-odd{$element} = 1    }
   return %is-odd.keys;

sub MAIN (*@inputs) {
   @inputs = 2, 5, 4, 4, 5, 5, 2 unless @inputs;
   put odd-number(@inputs);

The translation in Perl is straightforward:

#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';

sub odd_number (@inputs) {
   my %is_odd;
   for my $element (@inputs) {
      if ($is_odd{$element}) { delete $is_odd{$element} }
      else                   { $is_odd{$element} = 1    }
   return keys %is_odd;

my @inputs = @ARGV ? @ARGV : (2, 5, 4, 4, 5, 5, 2);
say odd_number(@inputs);

I guess it’s everything at this point… stay safe everyone!