leetcode230 Kth Smallest Element in a BST

 1 """
 2 Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.
 3 Note:
 4 You may assume k is always valid, 1 ≤ k ≤ BST's total elements.
 5 Example 1:
 6 Input: root = [3,1,4,null,2], k = 1
 7    3
 8   / \
 9  1   4
10   \
11    2
12 Output: 1
13 Example 2:
14 Input: root = [5,3,6,2,4,null,null,1], k = 3
15        5
16       / \
17      3   6
18     / \
. 19     2. 4
 20 is    /
 21 is  . 1
 22 is  the Output:. 3
 23 is  "" " 
24  " "" 
25  stack preorder used, plus a determination statement
 26 is  IF len (RES) == K
 27  "" " 
28  # Definition for binary Tree A . Node 
29  # class the TreeNode: 
30  #      DEF the __init __ (Self, X): 
31 is  #          self.val X = 
32  #          self.left = None 
33 is  #          self.right None = 
34 is  
35  class Solution:
 36      DEFkthSmallest (Self, the root, K):
 37 [          Stack = []
 38 is          RES = []
 39          CUR = the root
 40          the while Stack or CUR:
 41 is              IF CUR:
 42 is                  stack.append (CUR)
 43 is                  CUR = cur.left
 44 is              the else :
 45                  = CUR stack.pop ()
 46 is                  res.append (Cur.Val)
 47                  IF len (RES) == K:   # traversal sequence, determines the middle of a sentence 
48                      return RES [-1 ]
49                 cur = cur.right