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Kth Smallest Element in a BST

Problem statement​

Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes in the tree.

Example 1:

Input: root = [3,1,4,null,2], k = 1Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3Output: 3

Constraints:

  • The number of nodes in the tree is n.
  • 1 <= k <= n <= 104
  • 0 <= Node.val <= 104

Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?

My solution​

/**
 * 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
 * @param {number} k
 * @return {number}
 */
var kthSmallest = function(root, k) {
    const nodes = [];
    
    function dfs(node) {
        if (node.left) {
            dfs(node.left)
        }
        nodes.push(node.val);
        if (node.right) {
            dfs(node.right)
        }
    }
    
    dfs(root)
    // console.log(nodes)
    
    return nodes[k - 1]
};