leetcode-230

leetcode-230

题目描述：

给定一个二叉搜索树，编写一个函数 kthSmallest 来查找其中第 k 个最小的元素。

解法一，其实就是中序遍历，刚开始不知道怎么统计到第k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:
    def kthSmallest(self, root: TreeNode, k: int) -> int:
        def mid(root,k):
          if not root:
           return []
          return mid(root.left,k-1) + [root.val] + mid(root.right,k-1)
        return mid(root,k)[k-1]

解法二：直接统计到第k个数即可，始终没有return操作；

# 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:
      self.res = 0
      self.cnt = 0
      self.mid(root,k)
      return self.res
      
    def mid(self,root,k):
      if root.left:
        self.mid(root.left,k)
      self.cnt += 1
      if self.cnt == k:
        self.res = root.val
      
      if root.right:
        self.mid(root.right,k)