Last visitor

You are given an integer array @ints where each element is either a positive integer or -1.

Process the array from left to right while maintaining two lists:
• @seen stores previously seen positive integers (newest at the front)
• @ans stores the answers for each -1

Rules:
If $ints[$i] is a positive number insert it at the front of @seen
If $ints[$i] is -1:
... Let $x be how many -1s in a row we’ve seen before this one.
... If $x < length(@seen) append $seen[$x] to @ans
... Else append -1 to @ans

At the end, return @ans.

Examples

Example 1
Input: @ints = (5, -1, -1)
Output: (5, -1)
@seen = (5)
First  -1: @ans = (5)
Second -1: @ans = (5, -1)

Example 2
Input: @ints = (3, 7, -1, -1, -1)
Output: (7, 3, -1)
@seen = (3, 7)
First  -1: @ans = (7)
Second -1: @ans = (7, 3)
Third  -1: @ans = (7, 3, -1)

Example 3
Input: @ints = (2, -1, 4, -1, -1)
Output: (2, 4, 2)

Example 4
Input: @ints = (10, 20, -1, 30, -1, -1)
Output: (20, 30, 20)

Example 5
Input: @ints = (-1, -1, 5, -1)
Output: (-1, -1, 5)

Analysis

This was a difficult challenge to get my head around!

Eventually I deduced that to match the examples, the line

If $x < length(@seen) append $seen[$x] to @ans

should read append $seen[$#seen - $x] to @ans. In other words, the element to be appended is not the $xth element of @seen, but the $xth element counting back from the end.

Alternatively, perhaps 'newest at the front' means adding successive elements to the start of @seen - like Perl unshift - but that does not match the @seen shown for example 2.

Once I figured that out, the coding just follows the challenge text.

I have yet to discover a real-world use case for this algorithm!

A simple, effective and useful Perl implementation that uses little code to accurately address both issues. Without superfluous abstraction or flourish, Peter concentrates on the fundamental algorithmic requirements. It is an example of "get the job done" programming.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2025-10-27
use utf8;     # Week 345 - task 2 - Last visitor
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

last_visitor(5, -1, -1);
last_visitor(3, 7, -1, -1, -1);
last_visitor(2, -1, 4, -1, -1);
last_visitor(10, 20, -1, 30, -1, -1);
last_visitor(-1, -1, 5, -1);

sub last_visitor {
    
    my (@ints, @ans, @seen, $i, $x);
    
    # initialise
    @ints = @_;
    $x = 0;
    
    for $i (0 .. $#ints) {
        
        # if $ints[i] is +ve push it onto @seen
        if ($ints[$i] > 0) {
            push @seen, $ints[$i];
            $x = 0;

        # if $ints[$i] is -1 ...
        } elsif ($ints[$i] == -1) {
            
            # ... and $x <= length of @seen
            if ($x <= $#seen) {
                
                # push $x'th from last element of @seen onto @ans 
                push @ans, $seen[$#seen - $x];          
            } else {
                
                #  else push -1 onto @ans
                push @ans, -1;
            }
            
            # increment no of consecutive -1s
            $x ++;
        }
    }
    say qq[\nInput:  (] . join(', ', @ints) . ')';
    say qq[Output: (] . join(', ', @ans) . ')';
}

16 lines of code

Output from script

Input:  (5, -1, -1)
Output: (5, -1)

Input:  (3, 7, -1, -1, -1)
Output: (7, 3, -1)

Input:  (2, -1, 4, -1, -1)
Output: (2, 4, 2)

Input:  (10, 20, -1, 30, -1, -1)
Output: (20, 30, 20)

Input:  (-1, -1, 5, -1)
Output: (-1, -1, 5)