A general kind of solution in DFS problems is to pass an additional variable parent. In this case, we need to pass one more variable grandparent when doing the DFS process.
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def sumEvenGrandparent(self, root: TreeNode) -> int:
self.even_sum = 0
self._dfs(root, None, None)
return self.even_sum
def _dfs(self, node, parent, grandparent):
if node is None:
return
if grandparent and grandparent.val % 2 == 0:
self.even_sum += node.val
self._dfs(node.left, node, parent)
self._dfs(node.right, node, parent)