2.validate-binary-search-tree.md

Validate Binary Search Tree

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:

Input:
    2
   / \
  1   3
Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value
             is 5 but its right child's value is 4.

JavaScript

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isValidBST = function (root) {

    return isValidBSTHelper(root, Number.MIN_SAFE_INTEGER, Number.MAX_SAFE_INTEGER);
};

var isValidBSTHelper = function (root, min, max) {
    if (root === null) {
        return true;
    }

    if (root.val < min || root.val > max) {
        return false;
    }

    if (!isValidBSTHelper(root.left, min, root.val - 1) || !isValidBSTHelper(root.right, root.val + 1, max)) {
        return false;
    }
    
    return true;
}