297. 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.

For example, you may serialize the following tree

    1
   / \
  2   3
     / \
    4   5

as "[1,2,3,null,null,4,5]", just the same as how LeetCode OJ 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.

Solution: DFS

class Codec:
    def serialize(self, root):
        """
        Encodes a tree to a single string.
        :type root: TreeNode
        :rtype: str
        """
        vals = []

        def dfs(node):
            if not node:
                vals.append('#')
                return 
            vals.append(str(node.val))
            dfs(node.left)
            dfs(node.right)

        dfs(root)
        return ' '.join(vals)

    def deserialize(self, data):
        """
        Decodes your encoded data to tree.
        :type data: str
        :rtype: TreeNode
        """
        vals = data.split()
        vals.reverse()

        def dfs():
            if not vals:
                return None
            val = vals.pop()
            if val == '#':
                return None
            node = TreeNode(int(val))
            node.left = dfs()
            node.right = dfs()
            return node

        return dfs()