1001. A+B Format (20)

/*
https://www.patest.cn/contests/pat-a-practise/1001
1001. A+B Format (20)
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
Each input file contains one test case. Each case contains a pair of integers 
a and b where -1000000 <= a, b <= 1000000. The numbers are separated by a space.
Output
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
题目翻译： 
1001.A+B的标准格式
计算a+b，然后以标准格式输出。标准格式是指数字必须被分成三个一组，
用逗号分隔（除非少于4个数字）。 
输入
每个输入文件包含一个测试实例。每个实例包含一对整数a与b，a与b的范围是：
-1000000 <= a, b <= 1000000。数字用空格隔开。 
输出
对于每个测试实例，你需要在一行内输出a与b的和。和必须以标准格式输出。
样例输入
-1000000 9 
样例输出
-999,991

 */

#include <stdio.h>
int main ()
{
    int a, b;
    scanf("%d%d", &a, &b);
    int sum = a + b;
    if (sum < 0) {//负数输出方式
        printf("-");
        sum = -sum;
    }else if (sum == 0) {
        printf("0");
    }
    
    char str[20];
    int count = 1, tmp;
    while (sum > 0) {
        tmp = sum %10;//从低位1个1个开始分离出来 
        sum /= 10;
        str[count ++] = tmp + '0';//字符数组存储数字 
        if (sum != 0 && count % 4 == 0)
            str[count ++] = ',';//字符数组存储',' 占1个位置 
    }
    count --;
    
    for (int i = count; i >= 1; i--)
        printf("%c", str[i]);  //从高位到低位输出 
}