三、阅读程序-2

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#include <iostream>
    #include <string>
    using namespace std;

    char base[64];
    char table[256];

    void init(){
        for (int i = 0; i < 26; i++) base[i] = 'A' + i;
        for (int i = 0; i < 26; i++) base[26 + i] = 'a' + i;
        for (int i = 0; i < 10; i++) base[52 + i] = '0' + i;
        base[62] = '+', base[63] = '/';

        for (int i = 0; i < 256; i++) table[i] = 0xff;
        for (int i = 0; i < 64; i++) table[base[i]] = i;
        table['='] = 0;
    }

    string decode(string str)
    {
        string ret;
        int i;
        for (i = 0; i < str.size(); i += 4) {
            ret += table[str[i]] << 2 | table[str[i + 1]] >> 4;
            if (str[i + 2] != '=')
                ret += (table[str[i + 1]] & 0x0f) << 4 | table[str[i + 2]] >> 2;
            if (str[i + 3] != '=')
                ret += table[str[i + 2]] << 6 | table[str[i + 3]];
        }
        return ret;
    }

   int main()
   {
        init();
        cout << ((int)table[0])<< endl;

        string str;
        cin >> str;
        cout<<decode(str)<< endl;
        return 0;
    }