/in/foo.cc: In member function 'void BigInt::multiplyByDigit(int)':
/in/foo.cc:23:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i < digits.size(); ++i) {
                         ~~^~~~~~~~~~~~~~~
/in/foo.cc: In member function 'void BigInt::add(const BigInt&)':
/in/foo.cc:40:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             int sum = (i < digits.size() ? digits[i] : 0) +
                        ~~^~~~~~~~~~~~~~~
/in/foo.cc:41:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
                 (i < other.digits.size() ? other.digits[i] : 0) + carry;
                  ~~^~~~~~~~~~~~~~~~~~~~~
/in/foo.cc: In member function 'BigInt BigInt::operator*(const BigInt&) const':
/in/foo.cc:92:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i < other.digits.size(); ++i) {
                         ~~^~~~~~~~~~~~~~~~~~~~~