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

For example, given the following tree:

    1    
   / \   
  2   5  
 / \   \ 
3   4   6 

The flattened tree should look like:

1  
 \   
  2    
   \     
    3      
     \      
      4        
       \         
        5
         \           
          6

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def flatten(self, root: TreeNode) -> None:
        """
        Do not return anything, modify root in-place instead.
        """
        node, stack = root, []
        
        while node:
            if node.right:
                stack.append(node.right)
            node.right = node.left
            node.left = None
            
            if not node.right and stack:
                temp = stack.pop()
                node.right = temp
            
            node = node.right