230. 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?

Hint:

Try to utilize the property of a BST.

What if you could modify the BST node's structure?

The optimal runtime complexity is O(height of BST).

Related issue: 94. Binary Tree Inorder Traversal

/**
 * 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) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode node = root;
        while(!stack.isEmpty() || node != null){
            while(node != null){
                stack.push(node);
                node = node.left;
            }

            TreeNode top = stack.pop();
            if(--k == 0){
                node = top;
                break;
            }else{
                node = top.right;
            }
        }
        return node.val;
    }
}