#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

struct node {
  struct node *next;
  char data;
};

 
// duplicate should duplicate every node in the list, inserting the new
// node after the original node
// a z c d d..... a a z z c c d d d d
struct node *duplicate(struct node *head) { 
	struct node *tmp = head;
	while(tmp != NULL) {
		// make the new node
		struct node *new = malloc(sizeof(struct node));
		new->data = tmp->data;
		new->next = tmp->next;
		tmp->next = new;
		tmp = tmp->next->next;

	}
	
	
	return head;
}

////////////////////////////////////////////////////////////////////////
//               DO NOT CHANGE THE CODE BELOW                         //
////////////////////////////////////////////////////////////////////////
void print_list(struct node *head);
struct node *strings_to_list(int len, char *strings[]);

// DO NOT CHANGE THIS MAIN FUNCTION
int main(int argc, char *argv[]) {
	// create linked list from command line arguments
	struct node *head = strings_to_list(argc - 1, &argv[1]);

	// If you're getting an error here,
	// you have returned an uninitialized value
	struct node *new_head = duplicate(head);
	print_list(new_head);

	return 0;
}

// DO NOT CHANGE THIS FUNCTION
// create linked list from array of strings
struct node *strings_to_list(int len, char *strings[]) {
	struct node *head = NULL;
  	for (int i = len - 1; i >= 0; i = i - 1) {
		struct node *n = malloc(sizeof(struct node));
		assert(n != NULL);
		n->next = head;
		n->data = strings[i][0];
		head = n;
  	}
  	return head;
}

// print linked list
void print_list(struct node *head) {
	printf("[");

  	for (struct node *n = head; n != NULL; n = n->next) {
    	// If you're getting an error here,
   	 	// you have returned an invalid list
    	printf("%c", n->data);
    	if (n->next != NULL) {
      		printf(", ");
    	}
  	}
  	printf("]\n");
}