A61580. 2021年CSP-S提高组初赛阅读程序题： #include <iostream>

2021年CSP-S提高组初赛阅读程序题：

 #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 encode(string str)
 { 
    string ret;
    int i;
    for (i = 0; i + 3 <= str.size(); i += 3) {
        ret += base[str[i] >> 2];
        ret += base[(str[i] & 0x03) << 4 | str[i + 1] >> 4];
        ret += base[(str[i + 1] & 0x0f) << 2 | str[i + 2] >> 6];
        ret += base[str[i + 2] & 0x3f];
    }
    if (i < str.size()) {
        ret += base[str[i] >> 2];
        if (i + 1 == str.size()) {
            ret += base[(str[i] & 0x03) << 4];
            ret += "==";
        }
        else {
            ret += base[(str[i] & 0x03) << 4 | str[i + 1] >> 4];
            ret += base[(str[i + 1] & 0x0f) << 2];
            ret += "=";
        } 
    }
    return ret;
 }
 
 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;
 }

假设输入总是合法的（一个整数和一个不含空白字符的字符串，用空格隔开），完成下面的判断题和单选题：