设输入字符串长度为 n，decode 函数的时间复杂度为（ ）。
#include <iostream>
#include <string>
using namespace std;
char base[64];
char table[256];
vold 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;
}
void decode(string str)
{
string ret = ans;
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;
}