230 Kth Smallest Element in a BST

Python

class Solution:
    def kthSmallest(self, root, k):
        """
        :type root: TreeNode
        :type k: int
        :rtype: int
        """
        def inorder(r):
            return inorder(r.left) + [r.val] + inorder(r.right) if r else []

        return inorder(root)[k - 1]

Java

class Solution {
  public ArrayLinirst<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);
  }