Binary Tree Extract Leaves to Double Linked List LeetCode (Binary Tree To Doubly Linked List)

Problem: Given a Binary Tree, extract all leaves of it in a Doubly Linked List (DLL). Note that the DLL need to be created in-place. Assume that the node structure of DLL and Binary Tree is same, only the meaning of left and right pointers are different. In DLL, left means previous pointer and right means next pointer.  

Let the following be input binary tree
        1
     /     \
    2       3
   / \       \
  4   5       6
 / \         / \
7   8       9   10


Output:
Doubly Linked List
785910

Modified Tree:
        1
     /     \
    2       3
   /         \
  4           6 

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Solution: