114. Flatten Binary Tree to Link

# 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.
        """
        if not root:
            return root
        self.flattenHelper(root)
        
    def flattenHelper(self,root):
        if not root.left and not root.right:
            return root 
        left,right=root.left, root.right
        end=root 
        if left:
            root.left=None
            root.right=left
            end=self.flattenHelper(left)
            #print end.val
        if right:
            end.right=right
            end=self.flattenHelper(right)
            #print end.val
            
        return end