29. 最长有效括号

<?php
class Solution {
    /**
     * '(()' -> 2
     * ')()())' -> 4
     * @param $s
     * @return int|mixed
     */
    public function longestValidParentheses($s) {
        $max = 0;
        $start = 0;
        $stack = new SplStack();
        for ($i = 0; $i < strlen($s); $i++) {
            if ($s[$i] == '(') {
                $stack->push($i);
            } else {
                if ($stack->isEmpty()) {
                    $start = $i + 1;
                } else {
                    $stack->pop();
                    $max = $stack->isEmpty() ? max($max, $i - $start + 1) : max($max, $i - $stack->top());
                }
            }
        }
        return $max;
    }
}
$s = ')(((()())(';
$cls = new Solution();
$r = $cls->longestValidParentheses($s);
echo $r;