3. 无重复字符的最长字串

<?php
/**
 * 取给定字符串的最长子串
 *
 * abcabcbb
 * $array = [a => 0, b => 1, c => 2, ... , b => 6, b => 7];
 * @param string $s
 * @return int|mixed
 */
function lengthOfLongestSubString(string $s) {
    if (!$s || strlen($s) === 0) {
        return 0;
    }
    $array = [];
    $ret = 0;
    $start = 0;
    for ($i = 0; $i < strlen($s); $i++) {
        if (isset($array[$s[$i]]) && $start <= $array[$s[$i]]) {
            $start = $array[$s[$i]] + 1;
        } else {
            $ret = max($ret, $i - $start + 1);
        }
        $array[$s[$i]] = $i;
    }
    return $ret;
}
$str = 'abcabcbb';
$ret = lengthOfLongestSubString($str);
print_r($ret);