leetcode之Flatten Binary Tree to Linked List

这题要求就是在原有的基础上修改。将原来的tree修改成按照前序遍历的格式。采用的方法是先用一个栈存起来顺序，然后依次改变。代码如下：

# 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 = []
        def preorder(root):
            if not root:
                return root
            stack.append(root)
            if root.left:
                preorder(root.left)
            if root.right:
                preorder(root.right)
            return stack
        preorder(root)
        if not root:
            return root
        for i in range(len(stack) - 1):
            stack[i].left = None
            stack[i].right = stack[i + 1]