114. 二叉樹展開爲鏈表

class Solution:
    def flatten(self, root):
        """
        :type root: TreeNode
        :rtype: void Do not return anything, modify root in-place instead.
        """
        if root == None:
            return
        self.flatten(root.left)
        self.flatten(root.right)
        p = root
        if p.left == None:
            return
        p = p.left
        while p.right:
            p = p.right
        p.right = root.right
        root.right = root.left
        root.left = None