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Given the root of a binary tree, flatten the tree into a "linked list":
TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.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:
[0, 2000].-100 <= Node.val <= 100O(1) extra space)?take U forward