Validate binary search tree

# 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 __init__(self):
        self.prev = None
        self.is_valid = True
    def inorder(self, root):
        if not root or not self.is_valid:
            return None
        self.inorder(root.left)
        if self.prev is not None:
            if self.prev >= root.val:
                self.is_valid = False
        # print(self.prev, root.val)
        self.prev = root.val
        self.inorder(root.right)

    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        self.inorder(root)

        return self.is_valid

Validate Binary Search Tree

Difficulty: Medium

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

A valid 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.

 

Example 1:

Input: root = [2,1,3]
Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

 

Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • -231 <= Node.val <= 231 - 1