First Non-Repeating Value

Find the first input value whose final frequency is one.

Algorithm

Canonical input [3, 5, 2, 5, 3, 8, 2] prints 8. The replay uses the same input in every language, so this PHP DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.php

<?php
$arr = [3, 5, 2, 5, 3, 8, 2];
$count = [];
foreach ($arr as $value) {
    $count[$value] = ($count[$value] ?? 0) + 1;
}
foreach ($arr as $value) {
    if ($count[$value] === 1) {
        echo $value . PHP_EOL;
        break;
    }
}
?>

Complexity

Time: O(n) average
Space: O(k) for k distinct values

Implementation notes

Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
The trace highlights the hash table state after each write.

two-pass lookup The first pass builds a frequency table. The second pass keeps the original order and stops at the first value with frequency one.