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

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6
The flattened tree should look like:
   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

Hints:
If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.
'''

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


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


if __name__ == "__main__":
    None