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

To find the index of the first repeating character, iterate through the string while maintaining a hash map that records the first index at which each character appears. For each character, check if it already exists in the map. If it does, the current index is the second occurrence of a repeating character, and the stored index is the first occurrence. Return the stored index as the answer. If the loop completes without finding a repeat, return -1. This algorithm runs in O(n) time and uses O(1) additional space (since the map size is bounded by the alphabet). Example in PHP:

function firstRepeatingIndex(string $s): int {
    $map = [];
    for ($i = 0; $i < strlen($s); $i++) {
        $c = $s[$i];
        if (isset($map[$c])) {
            return $map[$c]; // first occurrence index
        }
        $map[$c] = $i;
    }
    return -1;
}

This returns the index of the first character that repeats, which is often what interviewers expect.