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

struct Node {
  int data;
  struct Node *next;
};

void append(struct Node *head, int data);
void printList(struct Node *head);
void insertAfter(struct Node *head, int key, int data);
void deleteLast(struct Node *head);
void deleteNode(struct Node *head, int key);

int main(int argc, char *argv[]) {

  struct Node *head = (struct Node *)malloc(sizeof(struct Node));
  printList(head);

  append(head, 1);
  append(head, 1);
  append(head, 2);
  append(head, 3);
  append(head, 5);
  append(head, 8);
  insertAfter(head, 3, 6);
  printList(head);
  deleteLast(head);
  printList(head);
  deleteNode(head, 2);
  printList(head);
  deleteNode(head, 9);

  return 0;
}

void append(struct Node *head, int data) {

  struct Node *newNode, *prev, *current = head;
  newNode = (struct Node *)malloc(sizeof(struct Node));
  newNode->data = data;

  while (current) {
    prev = current;
    current = current->next;
  }

  prev->next = newNode;
  newNode->next = current;
}

void printList(struct Node *head) {
  struct Node *current = head->next;

  if (!current) {
    printf("error: nothing to print, list is empty\n");
    return;
  }

  printf("head->");
  while (current) {
    printf("%i->", current->data);
    current = current->next;
  }
  printf("END\n");
}

void insertAfter(struct Node *head, int key, int data) {

  struct Node *current = head;

  while (current) {
    if (current->data == key)
      break;

    current = current->next;
  }

  if (!current) {
    printf("error: could not insertAfter. current is NULL\n");
    return;
  }

  struct Node *newNode = (struct Node *)malloc(sizeof(struct Node));
  newNode->data = data;

  newNode->next = current->next;
  current->next = newNode;
}

void deleteLast(struct Node *head) {
  if (!head->next) {
    printf("error: list is empty nothing to delete\n");
    return;
  }

  struct Node *prev = NULL, *current = head;

  while (current->next) {
    prev = current;
    current = current->next;
  }

  prev->next = NULL;
  free(current);
}

void deleteNode(struct Node *head, int key) {

  int found = 0;

  struct Node *prev = NULL, *current = head;
  while (current->next) {
    if (current->data == key) {
      found = 1;
      break;
    }
    prev = current;
    current = current->next;
  }

  // check
  if (!found) {
    printf("error: node %i not in list, nothing to delete\n", key);
    return;
  }
  //
  prev->next = current->next;
  free(current);
}