题目：

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 Specification:

Each input file contains one test case. Each case contains a pair of integers a and b where
−10​⁶≤a,b≤10^​6​ . The numbers are separated by a space.

Output Specification:

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

解题思路：

将两个数相加后的计算结果直接转换为 string 类型，然后从后往前遍历，每循环三次插入一个逗号。
虽然在 string 中间进行插入操作会消耗大量的时间，但题目中限定了数字的长度，因此不必考虑时间问题。

代码：

编译器：C++(g++)

#include <iostream>
#include <string>
using namespace std;

int main()
{
    int a,b;
    cin>>a>>b;
    int sum=a+b;
    string str=to_string(sum);
    int i=0;
    if('-'==str[0])
    {
        i=1;
    }
    for(int j=str.size()-1,count=0;j>i;--j)
    {
        ++count;
        if(3==count)
        {
            str.insert(j,",");
            count=0;
        }
    }
    cout<<str<<endl;
    return 0;
}