PAT甲级 1001 A+B Format

题目描述：
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

题目大意：
每三个数输出一个逗号（不包括最前头的负号，而且是从后往前每三个）。

代码如下：

#include <iostream>
using namespace std;
int main()
{
    int a,b,i=0;
    cin>>a>>b;
    string s=to_string(a+b);
    if(s[i]=='-')  cout<<s[i++];
    for(;i<s.size();i++){
        cout<<s[i];
       if((i+1)%3==s.size()%3&&i!=s.size()-1) cout<<",";
    }
    return 0;
}

来总结一下：
1）(i+1)%3==s.size()%3的意思就是，你第一次逗号出现时前面的个数得是你不足三位的个数是几个。（这个是从后往前三个输出一次的）
2）(i+1)%3 ==0是从前完后三个输出一次。