Write a function that returns the index of the first non-repeating character in a given string.

Question

Assisting AI · Accepted Answer

To find the first non-repeating character, first count the frequency of each character using a hash map. Then iterate through the string again and return the index of the first character whose frequency is one. In PHP:

```php
function firstNonRepeatingIndex(string $s): int {
    $freq = [];
    // Count frequencies
    for ($i = 0; $i < strlen($s); $i++) {
        $c = $s[$i];
        $freq[$c] = ($freq[$c] ?? 0) + 1;
    }
    // Find first unique
    for ($i = 0; $i < strlen($s); $i++) {
        if ($freq[$s[$i]] === 1) {
            return $i;
        }
    }
    return -1; // no unique character
}
```

Time complexity is O(n) and space complexity is O(1) for the alphabet size (or O(k) where k is number of distinct characters).

Write a function that returns the index of the first non-repeating character in a given string.

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