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Validate Binary Search Tree

Certification: Intermediate Level Accuracy: 0% Submissions: 0 Points: 10

Write a JavaScript program to determine if a given binary tree 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]
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  • Output: true
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  • Explanation: 
    • The tree has root node with value 2.
    • The left child has value 1, which is less than 2.
    • The right child has value 3, which is greater than 2.
    • Both subtrees are valid BSTs (single nodes).
    • Therefore, this is a valid BST.
    •
Example 2
    
  
  • Input: root = [5,1,4,null,null,3,6]
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  • Output: false
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  • Explanation: 
    • The tree has root node with value 5.
    • The left subtree contains node 1, which is valid.
    • The right subtree has root 4 with children 3 and 6.
    • Node 3 is in the right subtree of 5, but 3 < 5, violating BST property.
    • Therefore, this is not a valid BST.
    •
Constraints
    
  
  • The number of nodes in the tree is in the range [1, 10^4]
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  • -2^31 ≤ Node.val ≤ 2^31 - 1
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  • Time Complexity: O(n)
    • 
  
  • Space Complexity: O(n) for recursion stack
    •
RecursionTreePwCD. E. Shaw
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Hint
Solution

Solution Hints

    
  
  • Use recursive approach with min and max bounds for each node
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  • For each node, check if its value is within the valid range
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  • Update the bounds when recursing: left subtree gets updated max, right subtree gets updated min
    • 
  
  • The root node can have any value, so start with infinite bounds
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  • Return false immediately if any node violates the BST property
    •

Steps to solve by this approach:

 Step 1: Create a helper function that takes node, minimum bound, and maximum bound as parameters.
 Step 2: Handle base case - if node is null, return true (empty tree is valid BST).
 Step 3: Check if current node's value violates the min/max bounds, return false if so.
 Step 4: Recursively validate left subtree with updated maximum bound (current node's value).
 Step 5: Recursively validate right subtree with updated minimum bound (current node's value).
 Step 6: Return true only if both left and right subtrees are valid BSTs.
 Step 7: Start the validation with infinite bounds for the root node.

Submitted Code :