层数最深叶子节点的和

[yankewei]

给你一棵二叉树的根节点 root ，请你返回 层数最深的叶子节点的和 。

示例 1：

输入：root = [1,2,3,4,5,null,6,7,null,null,null,null,8]
输出：15

示例 2：

输入：root = [6,7,8,2,7,1,3,9,null,1,4,null,null,null,5]
输出：19

提示：

• 树中节点数目在范围 [1, 104] 之间。
• 1 <= Node.val <= 100

来源：力扣（LeetCode） 链接：https://leetcode.cn/problems/deepest-leaves-sum 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

[yankewei]

简单的树遍历

/**
 * 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 {

    private int $level = 0;
    private int $sum = 0;

    /**
     * @param TreeNode $root
     * @return Integer
     */
    function deepestLeavesSum(TreeNode $root) {
        $dfs = function (TreeNode $root, int $level) use (&$dfs): void {
            if ($root->left === null && $root->right === null) {
                if ($level > $this->level) {
                    $this->level = $level;
                    $this->sum = $root->val;
                } else if ($level === $this->level) {
                    $this->sum += $root->val;
                }
            }

            if ($root->left !== null) {
                $dfs($root->left, $level+1);
            }
            if ($root->right !== null) {
                $dfs($root->right, $level+1);
            }
        };

        $dfs($root, 0);
        return $this->sum;
    }
}