不用加减乘除实现加法

此处采用位运算，a+b无进位的结果和a^b相等，0 + 1 = 1 == 0 ^ 1 = 1
a+b的进位等于 (a&b) << 1 3+1的进位为 (11 & 1) << 1 = 10

public int add(int a, int b) {
        //进位为0时跳出
        while(b != 0){
            //进位
            int c = (a & b) << 1;
            //无进位和
            a = a^b;
            //进位
            b = c;
        }
        return a;
    }

运算3+1：

1. 1 != 0 进入循环 进位c = (11 & 1) << 1 = 10
   a = 11 ^ 1 = 10
   b = 10

2. 第二次循环 10 != 0
   c = (10 & 10) << 1 = 100
   a = 10 ^ 10 = 0
   b = 100

3. 第三次循环 100 ! = 0
   c = ( 0 & 100) << 1 = 0
   a = a ^ 100 = 100
   b = 0;

4. 结束 结果为 4