[LeetCode] 297. Serialize and Deserialize Binary Tree_hard tag: DFS, Divide and Conquer

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

 

Ideas:

利用preOrder traversal, (note, ans.append(str(root.val))), 因为我们要用“,”.join 的时候每个元素应该是str。来建立一个full tree, 也就是说如果没有children，那么用None来表示，

然后根据preOrder 的特性， data_queue[0] is root, data_queue[1] is left Child, data_queue[0] is right child, 然后recursive 去call就好。

T: O(n), S: O(n)

# 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
        """
        ans = []
        self.preOrder(root, ans)
        return ','.join(ans)
    
    def preOrder(self, root, ans):
        if not root:
            ans.append("#")
            return
        ans.append(str(root.val))
        self.preOrder(root.left, ans)
        self.preOrder(root.right, ans)

    def deserialize(self, data):
        """Decodes your encoded data to tree.
        
        :type data: str
        :rtype: TreeNode
        """
        preorder = collections.deque(data.split(','))
        return self.helper(preorder)
    
    def helper(self, preorder):
        if not preorder:
            return 
        root_val = preorder.popleft()
        if root_val == '#':
            return 
        root_val_num = int(root_val)
        root = TreeNode(root_val_num)
        root.left = self.helper(preorder)
        root.right = self.helper(preorder)
        return root