kth-smallest-element-in-a-bst.py

'''
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.

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?



'''



# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def kthSmallest(self, root: TreeNode, k: int) -> int:

        # Approach one
        if not root or k <= 0: return None
        def inorder(root):
            if not root: return []
            return inorder(root.left) + [root.val] + inorder(root.right)
        res = inorder(root)
        return res[k-1] if k <= len(res) else None



        # Approach two
        # if not root or k <= 0: return None
        # res , stack = [], []
        # while True:
        #     while root:
        #         stack.append(root)
        #         root = root.left
        #     if not stack:
        #         if k > len(res):
        #             return None
        #         else:
        #             return res[k-1]
        #     node = stack.pop()
        #     res.append(node.val)
        #     if node.right: root = node.right