404. Sum of Left Leaves

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     public $val = null;
 *     public $left = null;
 *     public $right = null;
 *     function __construct($val = 0, $left = null, $right = null) {
 *         $this->val = $val;
 *         $this->left = $left;
 *         $this->right = $right;
 *     }
 * }
 */
class Solution {

    /**
     * @param TreeNode $root
     * @return Integer
     */
    function sumOfLeftLeaves($root) {
        $sum = 0;
        $queue = [$root];
        while (!empty($queue)) {
            $node = array_shift($queue);
            if ($node->left) {
                $queue[] = $node->left;
                if ($node->left->left == null && $node->left->right == null) {
                    $sum += $node->left->val;
                }
            }

            if ($node->right) {
                $queue[] = $node->right;
            }
        }

        return $sum;
    }
}