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98. Validate Binary Search Tree

Tree Depth-First Search Binary Search Tree Binary Tree

Problem - 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 strictly less than the node's key.
  • The right subtree of a node contains only nodes with keys strictly 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

Solutions

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# 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]) -> bool:
        prev = float('-inf')

        def dfs(root: Optional[TreeNode]) -> bool:
            if root is None:
                return True
            if not dfs(root.left):
                return False

            nonlocal prev
            if prev >= root.val:
                return False
            prev = root.val
            return dfs(root.right)

        return dfs(root)

Submission Stats:

  • Runtime: 0 ms (100.00%)
  • Memory: 19.7 MB (68.01%)