Maximum Difference Between Node and Ancestor — LeetCode 1026 Python Solution

The problem

Given the root of a binary tree, find the maximum value v for which there exist different nodes a and b where v = |a.val - b.val| and a is an ancestor of b. A node a is an ancestor of b if either: any child of a is equal to b or any child of a is an ancestor of b.

Example

Input

root = [8,3,10,1,6,null,14,null,null,4,7,13]

Output

7

Explanation

We have various ancestor-node differences, some of which are given below :

Python solution

# 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:
        def dfs(root: Optional[TreeNode], mi: int, mx: int):
            if root is None:
                return
            nonlocal ans
            ans = max(ans, abs(mi - root.val), abs(mx - root.val))
            mi = min(mi, root.val)
            mx = max(mx, root.val)
            dfs(root.left, mi, mx)
            dfs(root.right, mi, mx)

        ans = 0
        dfs(root, root.val, root.val)
        return ans

Complexity

Pattern: Tree Traversal

Choose the order — preorder, inorder, postorder, level — and the problem solves itself. LeetCode 1026. Maximum Difference Between Node and Ancestor is filed here because LeetCode tags it Tree and Binary Tree, which is the vocabulary this hub collects.

The tree traversal guide has the Python template for the pattern and the 225 LeetCode problems that use it.

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