Flatten Binary Tree to Linked List

Leetcode problem 114; Difficulty: Medium

Problem Statement

Given the root of a binary tree, flatten the tree into a “linked list”:

• The “linked list” should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.
• The “linked list” should be in the same order as a pre-order traversal of the binary tree.

Constraints:

• The number of nodes in the tree is in the range [1, 2000].
• -100 <= Node.val <= 100

Example 1:

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

Output: [1,null,2,null,3,null,4,null,5,null,6]

Explanation:

There are two left leaves in the binary tree, with values 9 and 15 respectively.

Example 2:

Input: root = []

Output: []

Example 3:

Input: root = [0]

Output: [0]

Follow up

Can you flatten the tree in-place (with O(1) extra space)? (To be implemented)

Submitted Solution

The implemented solution received the following rating on Leetcode: