Serialize and Deserialize

Posted on |

Solution for Leetcode 449.

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 encode and decode BST.

leetcode 449 : Serialize and Deserialize BST
刚开始觉得怎么和271类似，于是呼写了个一毛一样的，居然过了。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
/**
 * 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) {
        if (!root) return "#";
        return to_string(root->val) + "," + serialize(root->left) + "," + serialize(root->right); 
    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        return help(data);
        
    }
private:
    
    TreeNode* help(string& data){
        if (data[0] == '#'){
            if (data.size() > 1){
                data = data.substr(2);
            }
            return nullptr;
        }
        auto root = new TreeNode(helper(data));
        root->left = help(data);
        root->right = help(data);
        return root;
    }
    
    int helper(string& data){
        int pos = data.find(',');
        int num = stoi(data.substr(0,pos));
        data = data.substr(pos + 1);
        return num;
    }
};
// Your Codec object will be instantiated and called as such:
// Codec codec;
// codec.deserialize(codec.serialize(root));

不小心点开了discuss里一个用c++的高票，果然牛逼啊。基本思路依旧是preorder遍历，但是inline，memcpy的应用再加上用4个char来代替string，以及利用binary serch tree 的特性免去了对string的解析和内存的浪费，按照这个思路重新写一下。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
class Codec {
public:
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        string res;
        dfs(root,res);
        return res;
    }
    // Decodes your encoded X`data to tree.
    TreeNode* deserialize(string data) {
        int pos = 0;
        return reconstruct(data, pos, INT_MIN, INT_MAX);
    }
private:
    inline void dfs(TreeNode* root,string &res){
        if(!root) return;
        char buf[4];
        memcpy(buf,&(root->val),sizeof(int));
        for(const auto& c : buf) res.push_back(c);
        dfs(root->left,res);
        dfs(root->right,res);
    }
    
    inline auto reconstruct(string data, int &pos, int min_v, int max_v) -> TreeNode*{
        if (pos >= data.size()) return nullptr;
        int val;
        memcpy(&val,&data[pos],sizeof(int));
        if (val > max_v || val < min_v){
            return nullptr;
        }
        auto root = new TreeNode(val);
        pos += sizeof(int);
        root->left = reconstruct(data,pos,min_v,val);
        root->right = reconstruct(data,pos,val,max_v);
        return root;
    }
};

又一次感受到了C++和C对底层性能的高效应用以及编译器的强大。