98-validate-binary-search-tree

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Description

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(Python)

# 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:
        return self.naive(root)

    # Time Complexity: O(n)
    # Space Complexity: O(h)
    def naive(self, root: Optional[TreeNode]) -> bool:
        def inorder(node):
            if node:
                inorder(node.left)
                res.append(node.val)
                inorder(node.right)

        res = []
        inorder(root)
        if len(res) == 2:
            return res[0] < res[1]
        for i in range(1, len(res) - 1):
            if not (res[i - 1] < res[i] < res[i + 1]):
                return False
        return True