## Description

https://leetcode.com/problems/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.

**Clarification:** The input/output 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.

**Example 1:**

Input:root = [1,2,3,null,null,4,5]Output:[1,2,3,null,null,4,5]

**Example 2:**

Input:root = []Output:[]

**Example 3:**

Input:root = [1]Output:[1]

**Example 4:**

Input:root = [1,2]Output:[1,2]

**Constraints:**

- The number of nodes in the tree is in the range
`[0, 10`

.^{4}] `-1000 <= Node.val <= 1000`

## Explanation

We can use breadth-first search in both serialize and deserialize. During serializing, we use breadth-first search to record all the node values into an array or a string in order. During deserializing, we follow a similar breadth-first search to creating the nodes and use an index pointer pointing to the next value in the array for creating a new node. In both serialize and deserialize processes, the queue would be only used to keep track of the nodes with values.

## Python Solution

```
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Codec:
def serialize(self, root):
"""Encodes a tree to a single string.
:type root: TreeNode
:rtype: str
"""
if not root:
return []
queue = []
queue.append(root)
results = []
while queue:
size = len(queue)
for i in range(size):
node = queue.pop(0)
if node:
results.append(node.val)
queue.append(node.left)
queue.append(node.right)
else:
results.append(None)
return results
def deserialize(self, data):
"""Decodes your encoded data to tree.
:type data: str
:rtype: TreeNode
"""
if not data:
return None
index = 0
root = TreeNode(data[index])
queue = []
queue.append(root)
index += 1
while queue:
size = len(queue)
for i in range(size):
node = queue.pop(0)
if data[index] != None:
node.left = TreeNode(data[index])
queue.append(node.left)
index += 1
if data[index] != None:
node.right = TreeNode(data[index])
queue.append(node.right)
index += 1
return root
# Your Codec object will be instantiated and called as such:
# ser = Codec()
# deser = Codec()
# ans = deser.deserialize(ser.serialize(root))
```

- Time complexity: O(N)
- Space complexity: O(N)

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