#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;

}

int main()

{

init();

cout << int(table[0]) << endl;

int opt;

string str;

cin >> opt >> str;

cout << (opt ? decode(str) : encode(str)) << endl;

return 0;

}

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：