101. Flatten Binary Tree to Linked List

MediumTree

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.

Examples

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

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

Explanation: Preorder traversal: 1,2,3,4,5,6.

Constraints

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