Recover Binary Search Tree, is a LeetCode problem from Tree subdomain. In this post we will see how we can solve this challenge in Swift

Problem Description

You can find the full details of the problem Recover Binary Search Tree at LeetCode


The key is to construct the connection between child & parent

  1. If cur.left == nil:

    • Output cur.val

    • Set cur = cur.right

  2. If cur.left != nil:

    • Find the precursor of cur, precursor

      i. If precursor.right == nil:

      • Set precursor.right = cur

      • Set cur = cur.left

      ii. If precursor.right != nil (which means precursor.right === cur):

      • Set precursor.right = nil (Recover the structure of original tree)

      • Output cur.val

      • Set cur = cur.right

  3. Repeat 1) & 2) until cur == nil

Time Complexity: O(n), Space Complexity: O(1)

Recommand Reading:

Definition for a binary tree node.

public class TreeNode {

public var val: Int

public var left: TreeNode?

public var right: TreeNode?

public init(_ val: Int) {

    self.val = val

    self.left = nil

    self.right = nil



Please check the main.swift snippet for the solution.

This solution originally posted at: Github by @soapyigu