题解 | #添加逗号#

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

int main() {
	char a[15];
	scanf("%s", a);
	int len = strlen(a);
	int sum = len;
	if (sum % 3 == 0)
	{
		sum -= 3;
	}
	sum /= 3;
	char* start = &a[len - 1];
	char* end = &a[len + sum - 1];
	a[len + sum] = '\0';
	while (sum--)
	{
		for (int i = 0; i < 3; i++)
		{
			*end = *start;
			end--;
			start--;
		}
		*end = ',';
		end--;
	}
	printf("%s", a);
	return 0;
}