题解 | #添加逗号#

#include <stdio.h>

int main() {
    long long n;
    char str[100] = {0};
    int count = 0;
    scanf("%lld", &n);
    for (int i = 0; n > 0; i++) {
        if (i % 4 == 3) {
            str[i] = ',';
        } else {
            str[i] = n % 10+'0';
            n /= 10;
        }

    }
    for (int i = strlen(str)-1; i >= 0; i--) {
        printf("%c", str[i]);
    }
    return 0;
}