1026.maximum-difference-between-node-and-ancestor.py

#
# @lc app=leetcode id=1026 lang=python3
#
# [1026] Maximum Difference Between Node and Ancestor
#

# @lc code=start
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def maxAncestorDiff(self, root: Optional[TreeNode]) -> int:
        # first intuition: dfs
        # 1. get the minimum child val recursively
        # 2. get the max diff
        # walk thourough all the node bottom up
        # return max diff
        if not root:
            return 0

        def helper(node, cur_max, cur_min):
            # if encounter leaves, return the max-min along the path
            if not node:
                return cur_max - cur_min
            # else, update max and min
            # and return the max of left and right subtrees
            cur_max = max(cur_max, node.val)
            cur_min = min(cur_min, node.val)
            left = helper(node.left, cur_max, cur_min)
            right = helper(node.right, cur_max, cur_min)
            return max(left, right)

        return helper(root, root.val, root.val)


# @lc code=end