98.Validate-Binary-Search-Tree.js

/**
 * 
 * Given a binary tree, determine if it is a valid binary search tree (BST).
 * 
 * Assume a 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:

    2
   / \
  1   3

Input: [2,1,3]
Output: true
Example 2:

    5
   / \
  1   4
     / \
    3   6

Input: [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.
 */

var isValidBST = function (
  root,
  lower = Number.MIN_SAFE_INTEGER,
  upper = Number.MAX_SAFE_INTEGER
) {
  if (!root) return true;

  let value = root.val;
  if (value <= lower || value >= upper) {
    return false;
  }

  if (!isValidBST(root.right, value, upper)) {
    return false;
  }

  if (!isValidBST(root.left, lower, value)) {
    return false;
  }
  return true;
};