98. Validate Binary Search Tree

Problem Statement

link: https://leetcode.com/problems/validate-binary-search-tree/ https://leetcode.cn/problems/validate-binary-search-tree/

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:

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

Input: root = [5,1,4,null,null,3,6] Output: false

Solution Approach

The solution utilizes a recursive approach, ensuring that every node’s value in the tree lies within the permitted range defined by its ancestors.

Algorithm

1. A recursive function, check, evaluates whether each node lies within a specific value range.
2. For each node, it confirms the node’s value is within its allowed range and then recursively checks its children, updating the range accordingly.
3. The tree is deemed valid if all nodes adhere to BST conditions; otherwise, it’s invalid.

Implement

    class Solution:
      def isValidBST(self, root: Optional[TreeNode]) -> bool:
          def check(root, l, r):
              if root == None:
                  return True
              val = root.val
              if val <= l or val >= r:
                  return False
              return check(root.left,l, root.val) and check(root.right, root.val, r)
          return check(root, float('-inf'), float('inf'))

    

    
    

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