Kth Smallest Element in a BST

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note: You may assume k is always valid, 1 ? k ? BST's total elements.

Follow up: What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

Credits:Special thanks to @ts for adding this problem and creating all test cases.

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
 public int kthSmallest(TreeNode root, int k) {
        List<Integer> result = new ArrayList<>();
        traverse(root, result);
        if (result.size() < k) return -1;
        else return result.get(k-1);
    }

    private void traverse(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }

        traverse(root.left, result);
        result.add(root.val);
        traverse(root.right, result);
    }

}