LeetCode-in-All

230. Kth Smallest Element in a BST

Medium

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

Example 1:

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

Output: 1

Example 2:

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

Output: 3

Constraints:

• The number of nodes in the tree is n.
• 1 <= k <= n <= 104
• 0 <= Node.val <= 104

Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?

Solution

; Definition for a binary tree node.
#|

; val : integer?
; left : (or/c tree-node? #f)
; right : (or/c tree-node? #f)
(struct tree-node
  (val left right) #:mutable #:transparent)

; constructor
(define (make-tree-node [val 0])
  (tree-node val #f #f))

|#

(define (inorder-traversal tree)
  (if (not tree)
      '()
      (append (inorder-traversal (tree-node-left tree))
              (list (tree-node-val tree))
              (inorder-traversal (tree-node-right tree)))))

(define (kth-smallest root k)
  (let ([inorder (inorder-traversal root)])
    (list-ref inorder (sub1 k))))

This site is open source. Improve this page.