1#include <iostream>2#include <string>3using namespace std;4 5char base[64];6char table[256];7 8void init()9{10    for (int i = 0; i < 26; i++) base[i] = 'A' + i;11    for (int i = 0; i < 26; i++) base[26 + i] = 'a' + i;12    for (int i = 0; i < 10; i++) base[52 + i] = '0' + i;13    base[62] = '+', base[63] = '/';14 15    for (int i = 0; i < 256; i++) table[i] = 0xff;16    for (int i = 0; i < 64; i++) table[base[i]] = i;17    table['='] = 0;18}19 20string encode(string str)21{22    string ret;23    int i;24    for (i = 0; i + 3 <= str.size(); i += 3) {25        ret += base[str[i] >> 2];26        ret += base[(str[i] & 0x03) << 4 | str[i + 1] >> 4];27        ret += base[(str[i + 1] & 0x0f) << 2 | str[i + 2] >> 6];28        ret += base[str[i + 2] & 0x3f];29    }30    if (i < str.size()) {31        ret += base[str[i] >> 2];32        if (i + 1 == str.size()) {33            ret += base[(str[i] & 0x03) << 4];34            ret += "==";35        }36        else {37            ret += base[(str[i] & 0x03) << 4 | str[i + 1] >> 4];38            ret += base[(str[i + 1] & 0x0f) << 2];39            ret += "=";40        }41    }42    return ret;43}44 45string decode(string str)46{47    string ret;48    int i;49    for (i = 0; i < str.size(); i += 4) {50        ret += table[str[i]] << 2 | table[str[i + 1]] >> 4;51        if (str[i + 2] != '=')52            ret += (table[str[i + 1]] & 0x0f) << 4 | table[str[i + 2]] >> 2;53        if (str[i + 3] != '=')54            ret += table[str[i + 2]] << 6 | table[str[i + 3]];55    }56    return ret;57}58 59int main()60{61    init();62    cout << int(table[0]) << endl;63 64    int opt;65    string str;66    cin >> opt >> str;67    cout << (opt ? decode(str) : encode(str)) << endl;68    return 0;69}

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：