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98. 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.

Example 1:

    2
   / \
  1   3
Input: [2,1,3]
Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6
Input: [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.

Solutions (Python)

1. Inorder Traversal

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

class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        nodes = []
        curr = root
        prev_val = float('-inf')

        while nodes or curr:
            while curr:
                nodes.append(curr)
                curr = curr.left

            curr = nodes.pop()

            if prev_val >= curr.val:
                return False
            prev_val = curr.val

            curr = curr.right

        return True