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.php

<?php
$arr = [2, 7, 11, 4, 5];
$target = 9;
$seen = [];
$first = -1;
$second = -1;
for ($i = 0; $i < count($arr); $i++) {
	$value = $arr[$i];
	$need = $target - $value;
	if (array_key_exists($need, $seen)) {
		$first = $seen[$need];
		$second = $i;
		break;
	}
	$seen[$value] = $i;
}
echo "[$first, $second]\n";

Complexity

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

Implementation notes

$arr is the pinned PHP array literal [2, 7, 11, 4, 5], and $target is 9.
$seen = [] is used as an associative array from value to index.
The loop is indexed: $i = 0; $i < count($arr); $i++, with $value = $arr[$i].
$need = $target - $value computes the complement before recording the current value.
Lookup uses array_key_exists($need, $seen) rather than truthiness, so a stored index 0 is still treated as present.
On a hit, $first = $seen[$need] and $second = $i, then break exits early.
On a miss, $seen[$value] = $i records the current value after the lookup.
The trace records a miss for 2 at index 0, storing {2: 0}, then a hit for value 7 needing 2.
echo "[$first, $second]\n"; prints the result pair as [0, 1].

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

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