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.

Example 1:

Input: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1
Output: 3

Solution

BST树的中序遍历序列是个递增的序列，我们只要取第k-1的元素即可。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def kthSmallest(self, root: TreeNode, k: int) -> int:
        self.k = k
        def helper(root):
            if root is None:
                return 0
            res= helper(root.left)
            if res:
                return res
            self.k = self.k-1
            if self.k ==0:
                return root.val
            else:
                return helper(root.right)
            
        return helper(root)

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Solution

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