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
SOLUTION:
# 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 isValidBST(self, root: Optional[TreeNode], minVal = float('-inf'), maxVal = float('inf')) -> bool:
if root:
if root.val <= minVal or root.val >= maxVal:
return False
if not self.isValidBST(root.left, minVal = minVal, maxVal = min(maxVal, root.val)) or not self.isValidBST(root.right, minVal = max(minVal, root.val), maxVal = maxVal):
return False
return True
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