10个数压缩成6个数表示

由0-9表示的数字串，现使其中4个数字不可用，用余下的数字进行重编码。写出编码和解码函数。

前缀编码，前5个剩下的编码用一个数字编码。最后一个以及不可用的4个数字，用两个数字进行编码。

 1 class Coding {
 2     public:
 3         void init(int uncode[], int n) {
 4             // get the code left
 5             vector<int> left;
 6             for (int i = 0, j = 0, m = 0; i < 10; ++i) {
 7                 if (j < 4 && i == uncode[j]) {
 8                     j++;
 9                     continue;
10                 }
11                 left.push_back(i);
12                 if (m < 5) {
13                     string code; code += (left[m++] + ‘0‘);
14                     codebook[i + ‘0‘] = code;
15                     decodebook[code] = i + ‘0‘;
16                 }
17             }
18
19             // code for the last one left
20             string code; code; code += (left.back() + ‘0‘);
21             code += (left.front() + ‘0‘);
22             codebook[left.back() + ‘0‘] = code;
23             decodebook[code] = left.back() + ‘0‘;
24
25             //generate codebook for uncode
26             for (int i = 0; i < 4; ++i) {
27                 string code; code += (left.back() + ‘0‘);
28                 code += (left[i + 1] + ‘0‘);
29                 codebook[uncode[i] + ‘0‘] = code;
30                 decodebook[code] = (uncode[i] + ‘0‘);
31             }
32             //for (map<char, string>::iterator it = codebook.begin(); it != codebook.end(); it++)
33             //  cout << it->first << " " << it->second << endl;
34         }
35
36         string encode(string &str) {
37             if (str.empty()) return "";
38             stringstream ans;
39             for(int i = 0; i < str.length(); ++i) {
40                 ans << codebook[str[i]];
41             }
42             return ans.str();
43         }
44
45         string decode(string &str) {
46             if (str.empty()) return "";
47             stringstream ans;
48             for(int i = 0; i < str.length(); ++i) {
49                 string tmp;
50                 tmp += str[i];
51                 if (decodebook.find(tmp) != decodebook.end() ||
52                         (++i < str.length() && decodebook.find(tmp += str[i]) != decodebook.end())){
53                     ans << decodebook[tmp];
54                 } else {
55                     return "error";
56                 }
57             }
58             return ans.str();
59         }
60     private:
61         map<string, char> decodebook;
62         map<char, string> codebook;
63 };

比如uncode={2,5,8,9},编码表就是这样子的：

 1 0 0
 2 1 1
 3 2 71
 4 3 3
 5 4 4
 6 5 73
 7 6 6
 8 7 70
 9 8 74
10 9 76

7用70来编码。