# Leetcode 114. Flatten Binary Tree to Linked List

[medium]

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.

Example 1:

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

Example 2:

`Input: root = []Output: []`

Example 3:

`Input: root = [0]Output: [0]`

Constraints:

• The number of nodes in the tree is in the range `[0, 2000]`.
• `-100 <= Node.val <= 100`

Follow up: Can you flatten the tree in-place (with `O(1)` extra space)?

[Think]

1. need to use recursion to find the deepest left node
2. this is reverse pre-order traversal
3. create a global TreeNode variable to track current right node

[Second- DFS- using stack]

1. adding travel node on current right side, let node.left become null

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CS new grad, 6 years experience related to supply chain management. Located in Bay area

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## Gary Chiang

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CS new grad, 6 years experience related to supply chain management. Located in Bay area