PAT (Advanced Level) Practice 1001 A+B Format （20 分)

题目链接：https://pintia.cn/problem-sets/994805342720868352/problems/994805528788582400

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 −. 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

#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>
#include <algorithm>
#include <stack>
using namespace std;
int main()
{
       int n,m,sum;
       while(cin>>n>>m){
           sum=n+m;
           if(sum==0){
               cout<<0<<endl;
               continue;
           }
           int flag=0;
           if(sum<0){
           flag=1;
           sum=-sum;
           } 
           char c='-';
           stack<char> s;
           int t=0;
           while(sum){
               c=sum%10+'0';
               s.push(c);
               t++;
               if(t%3==0&&sum/10) s.push(',');
               sum/=10;
           }
           if(flag) s.push('-');
           while(!s.empty()){
               cout<<s.top();
            s.pop(); 
           }
           cout<<endl;
       }
    return 0;
}