230. Kth Smallest Element in a BST

https://leetcode.com/problems/kth-smallest-element-in-a-bst/

JavaScript

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} k
 * @return {number}
 */
var kthSmallest = function(root, k) {
  const result = inorderTraversal(root)
  return result[k - 1]
};

function inorderTraversal(root, nodes, k) {
  if (!root) {
    return []
  }

  let result = []
  result = result.concat(inorderTraversal(root.left))
  result.push(root.val)
  result = result.concat(inorderTraversal(root.right))
  return result
}

JavaScript

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} k
 * @return {number}
 */
var kthSmallest = function(root, k) {
    const result = []
    const stack = []
    let current = root
    while (current || stack.length) {
        while (current) {
            stack.push(current)
            current = current.left
        }
        current = stack.pop()
        result.push(current.val)
        if (result.length === k) {
            return result[k - 1]
        }
        current = current.right
    }
};