python 3.x - Trying to validate Binary Search Tree using in-order traversal in Python3. What is wrong with my approach?

Question

Tried to solve https://leetcode.com/problems/validate-binary-search-tree/ problem with python.

My implementation looks something like this:

# 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
import math
class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        self.prev = -math.inf
        def in_order(root):
            if not root:
                return True
            in_order(root.left)
            if root.val <=self.prev:
                return False
            self.prev = root.val
            print(self.prev, root.val)
            in_order(root.right)
        return in_order(root)

It fails for the input [2,1,3] and returns False.

But if I change the code to:

# 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
import math
class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        self.prev = -math.inf
        def in_order(root):
            if not root:
                return True
            if not in_order(root.left):
                return False
            if root.val <=self.prev:
                return False
            self.prev = root.val
            print(self.prev, root.val)
            return in_order(root.right)
        return in_order(root)

It works!!!

Really baffled with the result. Please help me out!!!

Thanks in advance.

question from:https://stackoverflow.com/questions/66052242/trying-to-validate-binary-search-tree-using-in-order-traversal-in-python3-what

Answer 1

Waitting for answers