Medium — Binary Search Tree | Tree Traversal | Recursion | Inorder Traversal
The Problem
Find the kth smallest element in a Binary Search Tree (BST), where k is 1-indexed. The function should return the value of the kth smallest node when all nodes are sorted in ascending order.
Approach
This solution uses inorder traversal of the BST, which naturally visits nodes in sorted ascending order. It recursively traverses the left subtree, processes the current node, then traverses the right subtree, building a complete sorted list of all values before returning the kth element.
Time: O(n) · Space: O(n)
Code
class Solution:
def kthSmallest(self, root: TreeNode, k: int) -> int:
def inorder_traversal(node):
if not node:
return []
left = inorder_traversal(node.left)
right = inorder_traversal(node.right)
return left + [node.val] + right
inorder_values = inorder_traversal(root)
return inorder_values[k - 1]
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