题解 | #添加逗号#

#include <stdio.h>
#include <string.h>

#define N 15

char a[N];
char b[N * 2];

int main()
{
    scanf("%s", a);

    int k = 0;
    for (int i = strlen(a) - 1, j = 1; i >= 0; i -- , j ++ )
    {
        b[k ++ ] = a[i];
        if (j == 3 && i)
        {
            b[k ++ ] = ',';
            j = 0;
        }
    }

    for (int i = strlen(b) - 1; i >= 0; i -- ) printf("%c", b[i]);

    return 0;
}