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:

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode() {}
 8  *     TreeNode(int val) { this.val = val; }
 9  *     TreeNode(int val, TreeNode left, TreeNode right) {
10  *         this.val = val;
11  *         this.left = left;
12  *         this.right = right;
13  *     }
14  * }
15  */
16 class Solution {
17     public int sumEvenGrandparent(TreeNode root) {
18         return dfs(root, 1, 1);
19     }
20 
21     private int dfs(TreeNode root, int p, int gp){
22         if(root == null){
23             return 0;
24         }
25 
26         return (gp % 2 == 0 ? root.val : 0) + dfs(root.left, root.val, p) + dfs(root.right, root.val, p);
27     }
28 }