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LeetCode 98 Valid Binary Search Tree - Medium

98. Validate Binary Search Tree -- Medium

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98. Valid Binary Search Tree — Medium

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Problem

  1. Validate Binary Search Tree -- Medium

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:

2

/
1 3

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

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.

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: TreeNode) -> bool:
        lower = float('-inf')
        upper = float('inf')
        return self.dfs(root, lower, upper)
        
    def dfs(self, node, lower, upper):
        if not node:
            return True
        if node.val <= lower or node.val >= upper:
            return False
        
        if not self.dfs(node.left, lower, node.val):
            return False
        
        if not self.dfs(node.right, node.val, upper):
            return False
 
        return True

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