Leetcode 98 - Validate BST

Note:

  • Use inorder traversal.
  • If a tree is a BST, then its inorder traversal would be a strictly increasing array.

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.

Example
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Input: root = [2,1,3]
Output: true
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/**
* Definition for a binary tree node.
* function TreeNode(val, left, right) {
* this.val = (val===undefined ? 0 : val)
* this.left = (left===undefined ? null : left)
* this.right = (right===undefined ? null : right)
* }
*/
/**
* @param {TreeNode} root
* @return {boolean}
*/
var isValidBST = function(root) {
if (!root.left && !root.right) return true;
let inorder = [];
traverse(root, inorder);
let sorted = true;
for (let i = 1; i < inorder.length; i++) {
if (inorder[i] <= inorder[i-1]) sorted = false;
}
return sorted;

function traverse(node, inorder) {
if (!node) return;
node.left && traverse(node.left, inorder);
inorder.push(node.val);
node.right && traverse(node.right, inorder);
}
};