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Validate Binary Search Tree

Certification: Intermediate Level Accuracy: 0% Submissions: 0 Points: 15

Write a C 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, and both the left and right subtrees must also be binary search trees.

Example 1
    
  
  • Input: Tree structure [2,1,3]
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  • Output: true
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  • Explanation: 
    • Root node is 2, left child is 1, right child is 3. 
    • Left subtree contains only values less than 2 (which is 1). 
    • Right subtree contains only values greater than 2 (which is 3). 
    • Therefore, this is a valid BST.
    •
Example 2
    
  
  • Input: Tree structure [5,1,4,null,null,3,6]
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  • Output: false
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  • Explanation: 
    • Root node is 5, left child is 1, right child is 4. 
    • The right subtree has root 4, which is less than 5 (valid so far). 
    • Node 4 has left child 3 and right child 6. 
    • Node 3 is in the right subtree of root 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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  • You must validate the entire tree structure
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  • Time Complexity: O(n)
    • 
  
  • Space Complexity: O(h) where h is the height of the tree
    •
TreeGoldman SachsPhillips
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Hint
Solution

Solution Hints

    
  
  • Use in-order traversal to check if the values are in ascending order
    • 
  
  • Alternatively, use a recursive approach with min and max bounds
    • 
  
  • For each node, check if its value is within the valid range
    • 
  
  • Update the range for left and right subtrees accordingly
    • 
  
  • The left subtree should have values less than current node
    • 
  
  • The right subtree should have values greater than current node
    •

Steps to solve by this approach:

 Step 1: Define a helper function that takes a node and its valid range (min, max).
 Step 2: Check if the current node is NULL (base case - return true).
 Step 3: Validate if the current node's value is within the valid range.
 Step 4: If invalid, return false immediately.
 Step 5: Recursively validate the left subtree with updated max bound (current node's value).
 Step 6: Recursively validate the right subtree with updated min bound (current node's value).
 Step 7: Return true only if both left and right subtrees are valid BSTs.

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