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 nd{ 
  int key;
  struct nd *next;
  struct nd *prev;
};
typedef struct nd * ndPointer;
ndPointer nil;
ndPointer listSearch(int key){
  ndPointer i;
  for(i=nil;i->next!=nil;i=i->next){
  if(i->next->key==key)return i->next;
  }
  return NULL;
} 
void init(){
  nil = malloc(sizeof(struct nd));
  nil->next = nil; 
  nil->prev = nil;
}
void printList(){ 
  ndPointer 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(){ 
  ndPointer t = nil->next;
  if ( t == nil )return;
  deletend(t);
}
void deletend(ndPointer t){
  t->next->prev=t->prev;
  t->prev->next=t->next;
  free(t);
}
void deleteLast(){
  ndPointer i;
  i=nil->prev;
  deletend(i);
}
void delete(int key){
  ndPointer i; 
  i=listSearch(key);
  if(i!=NULL)deletend(i);
} 
void insert(int key){ 
  ndPointer x;
  x = malloc(sizeof(struct nd));
  x->key = key;
  x->next=nil->next;
  nil->next->prev=x;
  nil->next=x;
  x->prev=nil;
}
int main(){  
  char com[20]; 
  int np = 0, nd = 0,key, n, i,si = 0;
  scanf("%d", &n); 
  init();
  for ( i = 0; i < n; i++ ){
    scanf("%s%d", com, &key);
    if ( com[0] == 'i' ) {
      insert(key);
      np++;
      si++;
    } 
    else if ( com[0] == 'd' ) {
      if (strlen(com) > 6){
    if ( com[6] == 'F' ) deleteFirst();
    else if ( com[6] == 'L' ) deleteLast();
      }
      else { delete(key); nd++; } si--; }
  }
  printList(); 
  return 0;
}