LeetCode 1315. Sum of Nodes with Even-Valued Grandparent

原题链接在这里：https://leetcode.com/problems/sum-of-nodes-with-even-valued-grandparent/description/

题目：

Given the root of a binary tree, return the sum of values of nodes with an even-valued grandparent. If there are no nodes with an even-valued grandparent, return 0.

A grandparent of a node is the parent of its parent if it exists.

Example 1:

Input: root = [6,7,8,2,7,1,3,9,null,1,4,null,null,null,5]
Output: 18
Explanation: The red nodes are the nodes with even-value grandparent while the blue nodes are the even-value grandparents.

Example 2:

Input: root = [1]
Output: 0

Constraints:

• The number of nodes in the tree is in the range [1, 104].
• 1 <= Node.val <= 100

题解：

How does a node know its grandparent, we can pass in the parent and grandparent in the dfs call.

DFS returns the sum of subtree value whose grandparent is even.

Time Complexity: O(n).

Space: O(logn). stack space.

AC Java:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int sumEvenGrandparent(TreeNode root) {
        return dfs(root, 1, 1);
    }

    private int dfs(TreeNode root, int p, int gp){
        if(root == null){
            return 0;
        }

        return (gp % 2 == 0 ? root.val : 0) + dfs(root.left, root.val, p) + dfs(root.right, root.val, p);
    }
}