字符串最长不重复字串长度

class Solution
{

    /**
     * @param String $s
     * @return Integer
     */
    function lengthOfLongestSubstring($s)
    {
        $len = strlen($s);
        if ($len <= 1) {
            return $len;
        }

        $start = $ans = 0;
        $ss = '';
        $map = [];
        for ($end = 0; $end < $len; $end++) {
            if (isset($map[$s[$end]])) {
                $ss = substr($s, $start, $ans);
                $start = max($map[$s[$end]], $start);//这里很重要
            }
            $ans = max($ans, $end - $start + 1);
            $map[$s[$end]] = $end + 1;//加一很重要
            if ($ans > strlen($ss)) {
                $ss = substr($s, $start, $ans);
            }
        }

        var_dump($start, $end, $ans, $map, $ss);
    }
}

$a = new Solution();
$a->lengthOfLongestSubstring("abba");