449. Serialize and Deserialize BST——几乎所有树的面试题目都会回到BFS或者DFS，使用BFS，None节点存#

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

The encoded string should be as compact as possible.

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

DFS

# 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 root is None:
            return "#"
        return str(root.val) + " " + self.serialize(root.left) + " " + self.serialize(root.right)

    def deserialize(self, data):
        """Decodes your encoded data to tree.
        
        :type data: str
        :rtype: TreeNode
        """
        data = data.split()
        return self.deserialize_helper(data, [0])
        
    def deserialize_helper(self, data, i):
        if data[i[0]] == "#":
            return None
        root = TreeNode(int(data[i[0]]))
        i[0] += 1
        root.left = self.deserialize_helper(data, i)
        i[0] += 1
        root.right = self.deserialize_helper(data, i)
        return root
                
        

# Your Codec object will be instantiated and called as such:
# codec = Codec()
# codec.deserialize(codec.serialize(root))

 DFS 2:

# 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 root is None:
            return "#"
        return str(root.val) + " " + self.serialize(root.left) + " " + self.serialize(root.right)

    def deserialize(self, data):
        """Decodes your encoded data to tree.
        
        :type data: str
        :rtype: TreeNode
        """
        data = data.split()
        if data[0] == '#':
            return None
        node = root = TreeNode(int(data[0]))
        stack = [root]
        i = 1
        while data[i] != '#':
            node.left = TreeNode(int(data[i]))
            stack.append(node.left)
            node = node.left
            i += 1
        while stack:
            node = stack.pop()
            i += 1
            if data[i] != '#': 
                node.right = TreeNode(int(data[i]))
                stack.append(node.right)
                node = node.right
                i += 1
                while data[i] != '#':
                    node.left = TreeNode(int(data[i]))
                    stack.append(node.left)
                    node = node.left
                    i += 1
            else:
                node.right = None
        return root
        

# Your Codec object will be instantiated and called as such:
# codec = Codec()
# codec.deserialize(codec.serialize(root))

BFS

# 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 root is None:
            return ""
        stack = [root]
        ans = str(root.val)
        while stack:
            stack2 = []
            for node in stack:
                if node.left:
                    stack2.append(node.left)
                    ans += " "+str(node.left.val)
                else:
                    ans += " #"
                if node.right:
                    stack2.append(node.right)
                    ans += " " + str(node.right.val)
                else:
                    ans += " #"
            stack = stack2
        return ans
        

    def deserialize(self, data):
        """Decodes your encoded data to tree.
        
        :type data: str
        :rtype: TreeNode
        """
        array = data.split()
        if not array:
            return None
        root = TreeNode(int(array[0]))
        stack = [root]
        i = 1
        while stack:
            stack2 = []
            for node in stack:
                if array[i] != '#': 
                    node.left = TreeNode(int(array[i]))
                    stack2.append(node.left)
                else:
                    node.left = None
                i += 1
                if array[i] != '#': 
                    node.right = TreeNode(int(array[i]))
                    stack2.append(node.right)
                else:
                    node.right = None                
                i += 1
            stack = stack2
        return root
                
        

# Your Codec object will be instantiated and called as such:
# codec = Codec()
# codec.deserialize(codec.serialize(root))