LeetCode No.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.

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题目链接：https://leetcode.com/problems/serialize-and-deserialize-binary-tree/

题目大意：编写一个类，对一个二叉树进行序列化和反序列化，两个函数都只能使用其参数来构建。（序列化的意思就是：使用字符串储存二叉树，反序列化的意思是：使用你存储的字符串来构建原来的二叉树）

思路：序列化的做法有很多种，但是序列化和反序列化要选择一样的构建格式，就跟编码和译码的意思一样。

一般的做法有两种：

1、前序遍历

2、中序遍历

3、层次遍历

一般不会选择后续遍历，因为后序遍历的步骤比较复杂。

我这里选了我感觉最简单的那一种：层次遍历。

具体做法如下：

序列化：

1、用queue实现层次遍历

2、访问当前节点不为空时将该节点的值转成string放入结果中，用逗号‘,’隔开

3、如果当前节点左（右）子树不存在，则用‘n’表示，如果存在，则将该节点左（右）子树push进队列

反序列化：

1、同样用queue实现层次遍历

2、我们已知string中的数值是用逗号‘,’隔开的，而且‘n’表示空，因为每个节点有两个孩子（左右子树），所以我们每次从队列中取出一个节点都要计算它的左右子树。对每一次取值，首先判断当前字符是否为n，如果是n则下一节点（左或右子树）值为空，否则生成下一节点并push进队列

参考代码：

/** * Definition for a binary tree node. * 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) {        string ans = "" ;        if ( root == NULL )            return ans ;        ans += intToStr ( root -> val ) ;        queue <TreeNode*> q ;        q.push ( root ) ;        while ( !q.empty() )        {            TreeNode* temp = q.front() ;            q.pop() ;                        if ( temp -> left )            {            q.push ( temp -> left ) ;            ans += "," ;            ans += intToStr ( temp -> left -> val ) ;}            else            {            ans += ",n" ;            }                            if ( temp -> right )            {            q.push ( temp -> right ) ;            ans += "," ;            ans += intToStr ( temp -> right -> val ) ;}            else            {            ans += ",n" ;            }                        }        return ans ;    }    // Decodes your encoded data to tree.    TreeNode* deserialize(string data) {        int n = data.size() ;        if ( n == 0 )            return NULL ;        TreeNode* root = new TreeNode ( strToInt ( data ) ) ;        queue <TreeNode*> q ;        q.push ( root ) ;        while ( data.size() )        {            TreeNode* temp = q.front() ;            q.pop() ;            if ( data[0] == 'n' )            {                temp -> left = NULL ;                data.erase ( 0 , 1 ) ;                if ( data.size() && data[0] == ',' )                    data.erase ( 0 , 1 ) ;            }            else            {                temp -> left = new TreeNode ( strToInt ( data ) ) ;                q.push( temp -> left ) ;             }            if ( data.size() == 0 )                break ;            if ( data[0] == 'n' )            {                temp -> right = NULL ;                data.erase ( 0 , 1 ) ;                if ( data.size() && data[0] == ',' )                    data.erase ( 0 , 1 ) ;            }            else            {                temp -> right = new TreeNode ( strToInt ( data ) ) ;                q.push( temp -> right ) ;             }        }        return root ;    }private :    string intToStr ( int n )      {         if ( n == 0 )              return "0" ;          if ( n < 0 )              return "-" + intToStr ( -n ) ;          string ans = "" ;          while ( n )          {              ans = char ( n % 10 + '0' ) + ans ;              n /= 10 ;          }        return ans ;    }        int strToInt ( string& s )    {        int n = s.size() , ans = 0 ;        if ( n == 0 )            return 0 ;        if ( s[0] == '-' )        {            s.erase ( 0 , 1 ) ;            return - strToInt ( s ) ;        }        for ( int i = 0 ; i < n ; i ++ )        {            if ( s[i] == ',' )            {                s.erase ( 0 , i + 1 ) ;                return ans ;            }            else                ans = ans * 10 + s[i] - '0' ;        }        s = "" ;        return ans ;    }};// Your Codec object will be instantiated and called as such:// Codec codec;// codec.deserialize(codec.serialize(root));