PAT(Advanced Level) 1001 A+B Format (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 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

#include <iostream>
#include <cstdio>
using namespace std;
int main(){
    int x, y;
    scanf("%d %d", &x, &y);
    int sum = x + y;
    if(sum == 0){
        printf("0");
       return 0;
    }
    if(sum < 0){
        printf("-");
        sum = sum * (-1);
    }
    const int LEN = 8 + 2 + 1;
    char a[LEN];
    int i = 1;
    int j = 1;
    while(sum > 0){
        a[i++] = sum % 10 + '0';
        sum = sum / 10;
        if(j % 3 == 0 && sum > 0){
            a[i++] = ',';
        }
        ++j;
    }
    for(int k = i-1; k > 0; k--){
        printf("%c", a[k]);
    }

    return 0;
}