230. Kth Smallest Element in a BST

Score : P | Difficulty Medium


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

 

Example 1:

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

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

 

Constraints:

The number of elements of the BST is between 1 to 10^4.
You may assume k is always valid, 1 ≤ k ≤ BST's total elements.


M's answer with J's help of googling in order search:
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    public int kthSmallest(TreeNode root, int k) {
    ArrayList<Integer> values = new ArrayList<Integer>();
    inorder(root, values);
    return values.get(k - 1);
    }
    
    public void inorder(TreeNode root, ArrayList<Integer> myvalues) {
        if (root == null){
            return;
        }
        inorder(root.left, myvalues);
        myvalues.add(root.val);
        inorder(root.right, myvalues);
    }  
}