Kth Smallest Element in a BST

Problem

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

Example 1:

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

Example 2:

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

Constraints:

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

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 stack = [];
    while (stack.length || root) {
        while (root) {
            stack.push(root);
            root = root.left;
        }
        root = stack.pop();
        k--;
        if (k === 0) {
            return root.val;
        }
        root = root.right;
    }
};

We will implement a DFS solution (here we use an iterative version, but a recursive version would also work). Essentially, we do an inorder traversal to find the kth smallest value in the tree.

Problem​

Solution​

Problem

Solution