Day 20- Kth Smallest Element in a BST.java

/**
 * 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 ArrayList<Integer> inorder(TreeNode root, ArrayList<Integer> arr) {
    if (root == null) return arr;
    inorder(root.left, arr);
    arr.add(root.val);
    inorder(root.right, arr);
    return arr;
  }

  public int kthSmallest(TreeNode root, int k) {
    ArrayList<Integer> nums = inorder(root, new ArrayList<Integer>());
    return nums.get(k - 1);
  }
}