1001 A+B Format (20)（20 分）
Calculate a + b and output the sum in standard format -- that is, the digits must be separated into groups of three by commas (unless there are less than four digits).
Input
Each input file contains one test case. Each case contains a pair of integers a and b where -1000000 <= a, b <= 1000000. The numbers are separated by a space.
Output
For each test case, you should output the sum of a and b in one line. The sum must be written in the standard format.
Sample Input
-1000000 9
Sample Output
-999,991这里由于要在数字间加逗号，所以这里把数字和转化为字符串的形式（分离符号方便直接对数字操作，避免符号占1位导致后面位数判断出错）
注意：1、当数字>=0不需要+（=0：测试点4）
#include<iostream>
#include<string>
using namespace std;
int main() {
int a, b;
cin >> a >> b;
string ch = a + b >= 0 ? "" : "-"; //分离符号
string sum = to_string(abs(a + b));
int k = 1;
for (int i = sum.length() - 1; i >= 0; k++, i--) { //每3位加",",除首位
if (k % 3 == 0 && i)
sum.insert(i, ",");
}
cout << ch + sum; //输出数字
return 0;
}