Flatten Binary Tree to Linked List

Given a binary tree A, flatten it to a linked list in-place.

The left child of all nodes should be NULL.

1 <= size of tree <= 100000

First and only argument is the head of tree A.

Return the linked-list after flattening.

Input 1:

     1
    / \
   2   3

Input 2:

         1
        / \
       2   5
      / \   \
     3   4   6

Output 1:

1
 \
  2
   \
    3

Output 2:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

Explanation 1:

 Tree flattening looks like this.

Explanation 2:

 Tree flattening looks like this.