《剑指Offer（第二版）》面试题54. 二叉搜索树的第k大节点

给定一棵二叉搜索树，请找出其中第k大的节点。 

示例 1:
输入: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
输出: 4

示例 2:
输入: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1
输出: 4
 
限制：
1 ≤ k ≤ 二叉搜索树元素个数

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

class Solution:
    # 先右后左的中序遍历，遍历到第K就停止
    def kthLargest(self, root: TreeNode, k: int) -> int:
        index = 0
        stack = []
        node = root

        while stack or node:
            while node:
                stack.append(node)
                node = node.right
            node = stack.pop()
            index += 1
            if index == k:
                return node.val
            node = node.left