230. Kth Smallest Element in a BST

Problem

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 = 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

  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?

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) {
    var queue = [root];
    var num = 0;
    while (queue.length) {
        var node = queue.pop();
        node.right && queue.push(node.right);
        if (node.left) {
            queue.push(new TreeNode(node.val));
            queue.push(node.left);
        } else {
            num++;
            if (num === k) return node.val;
        }
    }
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).