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]