• Practice algorithmic problem solving

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.

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

Input: root = [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.

1. Rewrite the problem in your own words
2. Validate that you understand the problem
3. Write your own test cases
4. Pseudocode
5. Code!

And remember, don't run our tests until you've passed your own!

1. cd into the ruby folder
2. ruby <filename>.rb

1. cd into the javascript folder
2. node <filename>.js

1. cd into the ruby folder
2. bundle install
3. rspec

1. cd into the javascript folder
2. npm i
3. npm test