Calculate the sum of two integers a and b, but you are not allowed to use the operator + and -.

Given a = 1 and b = 2, return 3.

• 异或又被称其为“模2加法“
• 设置变量recipe模拟进位数字，模拟加法的实现过程

• a^b,求得结果
• a&b，求得进位
• 相加

public class Solution {
    public int getSum(int a, int b) {
        while (b != 0) {
            int c = a & b; //carry
            a ^= b; //add 
            b = c << 1;
        }
        return a;
    }
}