只适合这道题,循环写的

#include <stdio.h>

int main() {
    int count = 1;
    int n = 0;
    scanf("%d", &n);
    //计算一个数有几位数
    int num = n;
    while (num / 10)
    {
        ++count;
        num /= 10;
    }
    //得到一个数的每一位倒叙存入一个字符数组当中
    char arr[15] = { 0 };
    int i = 0;
    for (i = count; i >= 1; i--)
    {
        arr[i] = (n % 10)+48;
        n /= 10;
    }
    int number = 0;//满三加入一个逗号
    int d_count = 0;//向后移动的次数
    for (i = count; i >= 1; i--)
    {
        if (number != 3)
        {
            number++;
        }
        else
        {
            number = 0;
            int j = 0;
            //向后移动,给逗号腾出位置
            for (j = (count + d_count); j >= i + 1; j--)
            {
                char tmp = arr[j];
                arr[j] = arr[j + 1];
                arr[j + 1] = tmp;
            }
            arr[i + 1] = ',';
            number++;
            d_count++;
        }
    }
    for (i = 1; i <= (count + d_count); i++)
    {
        printf("%c", arr[i]);
    }
    return 0;
}