PAT-A 1001 A+B Format（Ｃ++）

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

不完备的题解：

#include
#include
#include
#include
#include
using namespace std;
#define MAX 0x3FFFFFFF
typedef long long  ll;


int main()
{
    ll a, b, c, c_tem;
    cin >> a >> b;
    c = a + b, c_tem = abs(c);
    if (c == 0) { cout << 0; return 0; }
    vector res;
    int sign = c / abs(c);
    while (c_tem != 0)
    {
        int tem = c_tem % 10;
        res.push_back(tem);
        c_tem = c_tem / 10;
    }

    if (sign == -1)
        cout << "-";
    int times = 0;
    while (res.size() != 0)
    {
        cout << res.back();
        res.pop_back(); times++;
        if (times % 3 == 0&& res.size() != 0)
            cout << ',';
    }
    return 0;
}

思路不难理解：先求得两数和，再求符号，拆成每个位并入栈，再弹栈，顺便打印逗号就行了。

测试了很多组数据，也考虑了结果为0的情况，不过不知道为什么只有部分正确