PHP 最长无重复子串长度

<?php
$str = "pwwke121w";
$existMap = [];
$longest = 0;
for(  $j=0, $i=0; $j < strlen($str); $j++ ){
    $char = $str[$j];
    $i = max($i , $existMap[$char]??0);
    $longest = max( $longest,$j - $i + 1 ) ;
    $existMap[$char] = $j +1 ;
}

echo $longest;