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.
Time: O(N)
Space: O(N)
1 # Definition for a binary tree node. 2 # class TreeNode: 3 # def __init__(self, x): 4 # self.val = x 5 # self.left = None 6 # self.right = None 7 8 class Solution: 9 def isValidBST(self, root: TreeNode) -> bool: 10 import sys 11 right_bound = sys.maxsize 12 left_bound = -sys.maxsize - 1 13 return self.helper(root, left_bound, right_bound) 14 15 def helper(self, root, left_bound, right_bound): 16 if root is None: 17 return True 18 if root.val <= left_bound or root.val >= right_bound: 19 return False 20 # 1. left child's child cannot larger than current node val 21 # 2. right child's child cannot be small than current node val 22 return self.helper(root.left, left_bound, root.val) and self.helper(root.right, root.val, right_bound) 23 24 25 26