Find the index of the first non-repeating character in a string.

The goal is to locate the first character that appears exactly once in the string. A two-pass approach works efficiently:

1. First pass: Count the frequency of each character using a hash map.
2. Second pass: Iterate the string again and return the index of the first character whose count is 1.

This guarantees O(n) time and O(1) space (for a fixed alphabet). Example in PHP:

function firstNonRepeatingIndex(string $s): int {
    $freq = [];
    for ($i = 0; $i < strlen($s); $i++) {
        $c = $s[$i];
        $freq[$c] = ($freq[$c] ?? 0) + 1;
    }
    for ($i = 0; $i < strlen($s); $i++) {
        if ($freq[$s[$i]] === 1) {
            return $i;
        }
    }
    return -1; // no non-repeating character
}

Edge cases: empty string returns -1; case sensitivity matters unless specified otherwise. This solution is concise, runs in linear time, and is suitable for interview settings.