98. Validate Binary Search Tree
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:
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
class Solution:
def isValidBST(self, root: Optional[TreeNode]) -> bool:
if root is None:
return True
tree = self.in_order(root)
return tree == sorted(set(tree))
def in_order(self, root: Optional[TreeNode]) -> List[int]:
if root is None:
return []
return self.in_order(root.left) + [root.val] + self.in_order(root.right)
class TreeNode:
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right