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Grokking the Coding Interview Patterns in C++

Validate Binary Search Tree

Understand and apply the depth-first search algorithm to validate binary search trees by ensuring each node meets BST properties. Learn to implement solutions that check key ordering and subtree validity for coding interviews.

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Statement

Given the root of a binary tree, check whether it is a valid binary search tree (BST).

A binary tree is a valid BST if for every node:

  • 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 are valid BSTs.

Constraints:

  • −104≤-10^{4} \leq Node.data ≤104\leq 10^{4}

  • The tree contains nodes in the range [1,500][1, 500].

Examples

Understand the problem

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

Valid Binary Search Tree

1.

Is the following binary tree a valid binary search tree?

  9
 / \
2  10
   / \
  7  20
A.

True

B.

False

1 / 3

Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Sequence - Vertical
Drag and drop the cards to rearrange them in the correct sequence.
1
2
3
4
5
6

Try it yourself

Implement your solution in the following coding playground.

C++
usercode > main.cpp
// Definition for a binary tree node

// template<class T>

// class TreeNode {

// public:

//     T data;

//     TreeNode<T>* left;

//     TreeNode<T>* right;



//     TreeNode(const T data) : data(data), left(nullptr), right(nullptr) {}

// };



int ValidateBst(TreeNode<int>* root)

{

    // Replace this placeholder return statement with your code

    return false;

}
Validate Binary Search Tree