Leetcode (Python): Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

Solution:

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a boolean
    def isValidBST(self, root):
        return self.isValidBSTRec(root, float("-infinity"), float("infinity"))
    
    def isValidBSTRec(self, root, min, max):
        if root == None:
            return True
        if root.val<=min or root.val>=max:
            return False
        solution = self.isValidBSTRec(root.left, min, root.val)
        solution = solution and self.isValidBSTRec(root.right, root.val, max)
        return solution