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>
typedef struct node{
  int key;
  struct node *next;
  struct node *prev;
}Node;

Node *nil;

Node *listSearch(int key){
  Node *cur = nil->next;
  while(cur!= nil && cur->key != key){
    cur = cur->next;
  }
  return cur;
}

void init(){
  nil = (Node *)malloc(sizeof(Node));
  nil->next = nil;
  nil->prev = nil;
}

void deleteNode(Node *t){
  if(t == nil) return;
  t->prev->next = t->next;
  t->next->prev = t->prev;
  free(t);
}

void deleteFirst(){
  deleteNode(nil->next);
}

void deleteLast(){
  deleteNode(nil->prev);
}

void deleteKey(int key){
  deleteNode(listSearch(key));
}

void insert(int key){
  Node *x= (Node *)malloc(sizeof(Node));
  x->key = key;
  x->next = nil->next;
  nil->next->prev = x;
  nil->next  =x;
  x->prev = nil;
}

int main(){
  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{
        deleteKey(key);
        nd ++;
      }
      size--;
    }
  }
Node *cur = nil->next;
int isf = 0;
while(1){
  if(cur == nil) break;
  if(isf++>0)printf(" ");
  printf("%d", cur->key);
  cur=cur->next;
}
printf("\n");
  return 0;
}