[9:30 am, 26/8/2025] +91 98402 41277: #include <stdio.h>
#include <string.h>
#include <ctype.h>

struct Guest {
    char username[100];
    struct Guest *next;
};

int isValidUsername(char username[]) {
    for (int i = 0; username[i] != '\0'; i++) {
        if (!isalnum(username[i]))
            return 0;
    }
    return 1;
}

int main() {
    int count;
    scanf("%d", &count);

    struct Guest guests[1000]; 
    for (int i = 0; i < count; i++) {
        scanf("%s", guests[i].username);
        if (!isValidUsername(guests[i].username)) {
            printf("Invalid input\n");
            return 0;
        }
        if (i < count - 1)
            guests[i].next = &guests[i + 1];
        else
            guests[i].next = NULL;
    }

    struct Guest *current = &guests[0];
    while (current !…
[9:30 am, 26/8/2025] +91 98402 41277: #include <stdio.h>

int main() {
    int n;