Two-Sum with Hash Lookup

Walk the array once, storing seen values in a lookup table. When the complement is already present, the result indices are known.

Algorithm

Basic Implementation

basic.pl

use strict;
use warnings;
my @arr = (2, 7, 11, 4, 5);
my $target = 9;
my %seen;
my ($first, $second) = (-1, -1);
for (my $i = 0; $i < @arr; $i++) {
	my $value = $arr[$i];
	my $need = $target - $value;
	if (exists $seen{$need}) {
		$first = $seen{$need};
		$second = $i;
		last;
	}
	$seen{$value} = $i;
}
print "[$first, $second]\n";

Complexity

Time: O(n) average
Space: O(n)

Implementation notes

Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
The final output is intentionally small and deterministic for cross-language comparison.

execution replay The checked-in replay follows the language-neutral state table for `array-two-sum-hash`.

cross-language comparison This Perl DSA version keeps the same data and final output as every other DSA book in this wave.