leetcode 297. Serialize and Deserialize Binary Tree 二叉树的序列化和反序列化 + 深度优先遍历DFS

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.

题意很简答，就是完成二叉树的序列化和反序列化，序列化很容易错，也就是二叉树的遍历；对于反序列化，也就是把一个字符串转化为一个二叉树，这个还是用同样的思想去做，看了网上的一个答案，才明白原来是这么的简单。

先序遍历的递归解法和层序遍历的非递归解法。先来看先序遍历的递归解法，非常的简单易懂，我们需要接入输入和输出字符串流istringstream和ostringstream，对于序列化，我们从根节点开始，如果节点存在，则将值存入输出字符串流，然后分别对其左右子节点递归调用序列化函数即可。对于去序列化，我们先读入第一个字符，以此生成一个根节点，然后再对根节点的左右子节点递归调用去序列化函数即可，参见代码如下：

建议和leetcode 331. Verify Preorder Serialization of a Binary Tree 二叉树的前序序列验证 和 leetcode 654. Maximum Binary Tree 构造最大二叉树 一起学习

建议和leetcode 105. Construct Binary Tree from Preorder and Inorder Traversal 中前序构造BST 和 leetcode 106. Construct Binary Tree from Inorder and Postorder Traversal 中后序构造BST 和leetcode 449. Serialize and Deserialize BST 二叉搜索树BST的序列化和反序列化一起学习，疑问做法感觉类似

代码如下：

import java.util.Collections;
import java.util.LinkedList;
import java.util.Queue;

/*class TreeNode
{
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) { val = x; }
}
*/

/*
* 这个序列化和反序列化就是通过中序来实现
* 要记着这么做
* */
public class Codec
{
private final String delimiter = ",";
private final String nullNode = "#";

// Encodes a tree to a single string.
public String serialize(TreeNode root)
{
StringBuilder builder=new StringBuilder();
serialize(root,builder);
return builder.toString();
}

/*
*  中序遍历得到序列化的值
* */
private void serialize(TreeNode root, StringBuilder builder)
{
if(root==null)
builder.append(nullNode).append(delimiter);
else
{
builder.append(root.val).append(delimiter);
serialize(root.left, builder);
serialize(root.right, builder);
}
}

// Decodes your encoded data to tree.
public TreeNode deserialize(String data)
{
if(data==null || data.length()<=0)
return null;
String[] tmp = data.split(delimiter);
Queue<String> queue=new LinkedList<>();
Collections.addAll(queue, tmp);

TreeNode root=deserialize(queue);
return root;
}

private TreeNode deserialize(Queue<String> queue)
{
if(queue==null || queue.size()<=0)
return null;

String one=queue.poll();
if(one.equals(nullNode))
return null;
else
{
TreeNode root=new TreeNode(Integer.parseInt(one));
root.left=deserialize(queue);
root.right=deserialize(queue);
return root;
}
}
}

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

下面是C++的做法，这道题十分的棒们很值得学习

代码如下：

#include <iostream>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <queue>
#include <stack>
#include <string>
#include <climits>
#include <algorithm>
#include <sstream>
#include <functional>
#include <bitset>
#include <numeric>
#include <cmath>
#include <regex>

using namespace std;

/*
struct TreeNode
{
int val;
TreeNode *left;
TreeNode *right;
TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
*/

class Codec
{
public:

// Encodes a tree to a single string.
string serialize(TreeNode* root)
{
stringstream ss;
dfs1(ss,root);
return ss.str();
}

void dfs1(stringstream &ss,TreeNode* root)
{
if (root == NULL)
ss << "#" << " ";
else
{
ss << root->val << " ";
dfs1(ss,root->left);
dfs1(ss,root->right);
}
}

// Decodes your encoded data to tree.
TreeNode* deserialize(string data)
{
stringstream ss(data);
return dfs2(ss);
}

TreeNode* dfs2(stringstream &ss)
{
string one = "";
ss >> one;
if (one == "#")
return NULL;
else
{
TreeNode* root = new TreeNode(stoi(one));
root->left = dfs2(ss);
root->right = dfs2(ss);
return root;
}

}
};

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