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.
此题讲解全部都在bit manipulation里面：
public class Solution {
public int getSum(int a, int b) {
if(b==0) return a;
return getSum(a^b,(a&b)<<1);
}
}