LeetCode 297. Serialize and Deserialize Binary Tree

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, 104].
  • -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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