PAT 甲级 1001 A+B Format

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≤106
?? . 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 <string>
#include <algorithm>
using namespace std;

int n, m;

int main()
{
    cin >> n >> m;
    int k = n + m;
    string ans;
    int flag = 0;
    if (k < 0) {flag = 1;k = abs(k);}
    else if (k == 0) {cout << 0 << endl;return 0;}

    int count = 0;
    while (k != 0) {
        int tmp = k % 10;k = k / 10;
        ans += (char)(‘0‘ + tmp);
        count++;
        if (count % 3 == 0 && k!=0) {ans += ‘,‘;}
    }
    reverse(ans.begin(),ans.end());
    if (flag) ans.insert(ans.begin(), ‘-‘);

    cout << ans << endl;
    return 0;
}