19. 生成括号

<?php
class Solution {
    public $list = [];
    public function generateParenthesis($n) {
        $this->_gen(0, 0, $n, '');
        return $this->list;
    }
    private function _gen($left, $right, $num, $result) {
        echo $left . ' ' . $right . "\t";
        if ($left == $num && $right == $num) {
            array_push($this->list, $result);
            return;
        }
        if ($left < $num) {
            $this->_gen($left + 1, $right, $num, $result . '(');
        }
        if ($right < $num && $left > $right) {
            $this->_gen($left, $right + 1, $num, $result . ')');
        }
        return;
    }
}
$cls = new Solution();
$r = $cls->generateParenthesis(3);
print_r($r);