Serialize and Deserialize Binary Tree

Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment.

Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure.

Example: 

You may serialize the following tree:

    1
   / 
  2   3
     / 
    4   5

as "[1,2,3,null,null,4,5]"

Clarification: The above format is the same as how LeetCode serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

序列化和反序列化二叉树。解法主要是两种层序遍历和前序遍历。

前序遍历基本采用递归写法，需要保存一个全局变量来告诉代码，现在在处理数组的第几个数字。比较麻烦。

所以这里给出层序遍历的代码：

class Codec:

    def serialize(self, root):
        """Encodes a tree to a single string.
        
        :type root: TreeNode
        :rtype: str
        """
        if root is None:
            return ""
        res = []
        queue = collections.deque()
        queue.append(root)
        while queue:
            node = queue.popleft()
            if node:
                res.append(str(node.val))
                queue.append(node.left)
                queue.append(node.right)
            else:
                res.append('#')
        return ",".join(res)          

    def deserialize(self, data):
        """Decodes your encoded data to tree.
        
        :type data: str
        :rtype: TreeNode
        """
        if data == "":
            return None
        res = data.split(',')
        queue = collections.deque()
        root = TreeNode(res[0])
        i = 1
        queue.append(root)
        while queue:
            cur = queue.popleft()
            leftval = res[i]
            if leftval != '#':
                left = TreeNode(int(leftval))
                cur.left = left
                queue.append(left)
            i += 1
            rightval = res[i]
            if rightval != '#':
                right = TreeNode(int(rightval))
                cur.right = right
                queue.append(right)
            i += 1
        return root     

注意这里的写法对于[1,2,3,null,null,4,5]的序列化结果为“1,2,3,#,#,4,5，#，#，#，#”

同时注意这里的反序列化实际利用第一层只有根节点的信息，所以先放到queue中。