快速乘

作者：handsomehow
链接：https://www.nowcoder.com/discuss/69932?type=2&order=0&pos=39&page=1
来源：牛客网

int mul(int a, int b){
      int sig = 1;
      if(b < 0){
        sig *= -1;
          b = -b;
    }
      if(a < 0){
        sig *= -1;
          a = -a;
    }
    int ret = 0;
      while(b){
        if(b&1){
            ret = ret + a;
        }
          a += a;
          b >>= 1;
    }
      return ret * sig;
}