char *lltoa(long long val, char *buf, int radix);
lltoa converts val to a string in base radix and places the result in buf.
lltoa returns buf as the result.
If radix is greater than 36, the result is undefined.
If val is negative and radix is 10, the string has a leading minus sign (-); for all other values of radix, value is considered unsigned and never has a leading minus sign.