Arrays and Iteration

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

  • 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 PHP DSA version keeps the same data and final output as every other DSA book in this wave.