Leetcode 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?

本题和不用递归的方法中序遍历BST (Leetcode 94题) 基本上一样。我们只要在iteratively的过程中输出第K个val就可以了。

 class Solution(object):
     def kthSmallest(self, root, k):
         """
         :type root: TreeNode
         :type k: int
         :rtype: int
         """
         ans = []
         stack = []

         while stack or root:
             while root:
                 stack.append(root)
                 root = root.left
             root = stack.pop()
             ans.append(root.val)
             if len(ans) == k:
                 return ans[-1]
             root = root.right
         return ans[-1]