371. Sum of Two Integers

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

Example 1:

Input: a = 1, b = 2
Output: 3

Example 2:

Input: a = -2, b = 3
Output: 1

     异或运算(^)是不进位加法，找到进位，将其和异或的结果加起来即可。进位通过与运算（&）并左移一位得到。异或（^）的结果和进位的相加，还是要调用getSum这个函数。代码如下：

class Solution {
public:
  int getSum(int a, int b) {
	if (b == 0) return a;       
	int sum = a ^ b;        
	int carry = (a & b) << 1;       
	return getSum(sum, carry);
   }
};