230. Kth Smallest Element in a BST

Given a binary search tree, write a function kthSmallest to find the k th smallest element in it.

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

Follow up: What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

Constraints:

• The number of elements of the BST is between 1 to 10^4.
• You may assume k is always valid, 1 ≤ k ≤ BST's total elements.

# @lc code=start
using LeetCode

function kth_smallest_in_BST(root, k::Int)::Int
    isnothing(root) && return true
    arr = Int[]
    function pre_tra(root::TreeNode{Int})
        pre_tra(root.left)
        push!(arr, root.val)
        pre_tra(root.right)
    end
    pre_tra(::Nothing) = nothing
    pre_tra(root)
    arr[k]
end
# @lc code=end

kth_smallest_in_BST (generic function with 1 method)

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