Problem:
Doubly Linked List

Your task is to implement a double linked list.

Write a program which performs the following operations:

insert x: insert an element with key x into the front of the list.

delete x: delete the first element which has the key of x from the list. If there is not such element, you need not do anything.

deleteFirst: delete the first element from the list.

deleteLast: delete the last element from the list.

Input

The input is given in the following format:

n

command1

command2

...

commandn

In the first line, the number of operations n is given. In the following n lines, the above mentioned operations are given in the following format:

insert x

delete x

deleteFirst

deleteLast

Output

Print all the element (key) in the list after the given operations. Two consequtive keys should be separated by a single space.

Constraints

The number of operations ≤ 2,000,000

The number of delete operations ≤ 20

0 ≤ value of a key ≤ 109

The number of elements in the list does not exceed 106

For a delete, deleteFirst or deleteLast operation, there is at least one element in the list.

Sample Input 1

7
insert 5
insert 2
insert 3
insert 1
delete 3
insert 6
delete 5

Sample Output 1

6 1 2

Sample Input 2

9
insert 5
insert 2
insert 3
insert 1
delete 3
insert 6
delete 5
deleteFirst
deleteLast

Sample Output 2

1

Solution in C (judge verdict: Accepted)

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

struct node {
  unsigned int key;
  struct node *next;
  struct node *prev;
};

typedef struct node * NodePointer;

NodePointer nil;

NodePointer listSearch(int key) {
  NodePointer t;
  for (t = nil->next; t != nil; t = t->next) {
    if (t->key == key) {
      return t;
    }
  }
  return nil;
}

void init(void) {
  nil = malloc(sizeof(struct node));
  nil->next = nil;
  nil->prev = nil;
}

void printList(void) {
  NodePointer cur = nil->next;
  int isf = 1;
  while(1) {
    if (cur == nil) break;
    if (isf == 0)  printf(" ");
    printf("%d", cur->key);
    cur = cur->next;
    isf = 0;
  }
  printf("\n");
}

void deleteNode(NodePointer t){
  NodePointer a_del = t->next, b_del = t->prev;
  if (a_del == nil) {
    nil->prev = b_del;
    b_del->next = nil;
  } else if (b_del == nil) {
    nil->next = a_del;
    a_del->prev = nil;
  } else {
    a_del->prev = b_del;
    b_del->next = a_del;
  }
  free(t);
}

void deleteFirst(void) {
  NodePointer t = nil->next;
  if (t == nil) return;
  deleteNode(t);
}

void deleteLast(void) {
  NodePointer t = nil->prev;
  if (t == nil) return;
  deleteNode(t);
}

void delete(int key) {
  NodePointer t = listSearch(key);
  if (t == nil) return;
  deleteNode(t);
}


void insert(int key) {
  NodePointer x;
  x = malloc(sizeof(struct node));
  x->key = key;

  if (nil->prev == NULL) {
    nil->prev = x;
  }
  
  if (nil->next == NULL) {
    nil->next = x;
    x->prev = nil;
    x->next = nil;
  } else {
    nil->next->prev = x;
    x->next = nil->next;
    x->prev = nil;
    nil->next = x;
  }  
}

int main(void) {
  int key, n, i;
  int size = 0;
  char com[20];
  int np = 0, nd = 0;
  scanf("%d", &n);
  init();
  for (i = 0; i < n; i++) {
    scanf("%s%d", com, &key);
    if (com[0] == 'i') {
      insert(key); 
      np++; 
      size++;
    } else if (com[0] == 'd') {
      if (strlen(com) > 6) {
        if (com[6] == 'F') deleteFirst();
        else if (com[6] == 'L') deleteLast();
      } else {
        delete(key);
        nd++;
      }
      size--;
    }
  }

  printList();
  return 0;
}