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114. Flatten Binary Tree to Linked List


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]



  • 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)?

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