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

typedef struct node * NodePointer;

NodePointer nil;

NodePointer listSearch(int);
void init(void);
void printList(void);
void deleteFirst(void);
void deleteLast(void);
void delete(int);
void insert(int);

int main(){
  int n,i,num,np,nd,size;
  char order[11];

  init();
  size = np = nd = 0;

  scanf("%d" ,&n);
  for(i = 0 ; i < n ; i++){
    scanf("%s %d" ,order ,&num);

    if(order[0] == 'i'){
      insert(num);
      np++; 
      size++;
    }else if(order[0] == 'd'){
      if(strlen(order) > 6){
	if(order[6] == 'F'){
	  deleteFirst();
	}else if(order[6] == 'L'){
	  deleteLast();
	}
      }else{
	delete(num);
	nd++;
      }
      size--;
    }
  }
  printList();

  return 0;
}

NodePointer listSearch(int key){
  NodePointer x,y;

  y = nil;
  x = y->next;

  while(x != nil){
    if(x->key == key){
      return y;
    }
    y = x;
    x = y->next;
  }
  return NULL;
}

void init(){
  nil = malloc(sizeof(struct node));
  nil->next = nil;
  nil->prev = nil;
  nil->key = -1;
}

void printList(){
  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 deleteFirst(){
  NodePointer x,y;

  x = nil->next;
  y = x->next;
  y->prev = nil;
  nil->next = y;
  free(x);
}

void deleteLast(){
  NodePointer x,y;

  x = nil->prev;
  y = x->prev;
  y->next = nil;
  nil->prev = y;
  free(x);
}

void delete(int key){
  NodePointer x,y,z;

  if(nil->next->key == key){
    deleteFirst();
    return;
  }

  x = listSearch(key);
  if(x == NULL){
    return;
  }
  z = x->next;

  y = x->next->next;
  x->next = y;
  y->prev = x;
  
  free(z);
}

void insert(int key){
  NodePointer x,y;
  x = malloc(sizeof(struct node));
  x->key = key;
  
  y = nil->next;
  nil->next = x;
  x->prev = nil;
  x->next = y;
  y->prev = x;
}