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

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

在不考虑进位的情况下 a ^ b，只考虑进位的情况下 (a & b) << 1，这两个结果相加为和的结果

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