230. Kth Smallest Element in a BST

Given the root of a binary search tree, and an integer k, return the kth (1-indexed) smallest element in the tree.

Example 1:

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

Example 2:

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

inorder讓數列保持大小順序

# 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

T:O(n), S:O(n), inorder sorted 

class Solution:
    def kthSmallest(self, root: TreeNode, k: int) -> int:
        ans = []
        if not root: return
        
        def inorder(node):
            if not node or len(ans) == k: return 
            
            inorder(node.left)
            if len(ans)<k:
                ans.append(node.val)
                inorder(node.right)
            else: return
        
        inorder(root)
        return ans[-1]

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