98. Validate Binary Search Tree

2019-08-05

98. Validate Binary Search Tree

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:

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    2
   / \
  1   3

Input: [2,1,3]
Output: true

Example 2:

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    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.

Code:

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        return self.helper(root, float('-inf'), float('inf'))

    def helper(self, node, minVal, maxVal):
        if node == None:
            return True

        if node.val <= minVal or node.val >= maxVal:
            return False

        left = self.helper(node.left, minVal, node.val)
        right = self.helper(node.right, node.val, maxVal)

        return left and right
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