68: Flatten binary tree

Given a binary tree, return a flattened tree

Note: do it in-place, i.e. update the original tree rather than creating a new one

 

Example 1

Input:     1   
          / \  
         2   3 

Output:    1
          / \
       null  2
            / \
         null  3

Example 2

Input:      1
           / \
          2   5
         / \   
        3   4  

Output: 1 -> 2 -> 3 -> 4 -> 5