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,*pre;
};

struct Node *n;

struct Node* lserch(int key){
  struct Node *cur=n->next;
  while(cur != n && cur->key != key){
    cur=cur->next;
  }
  return cur;
}

void init() {
  n=(struct Node *)malloc(sizeof(struct Node));
  n->next=n;
  n->pre=n;
}

void list() {
  struct Node *cur = n->next;
  int is=0;
  while (1) {
    if(cur==n) break;
    if(is++ > 0) printf(" ");
    printf("%d",cur->key);
    cur=cur->next;
  }
  printf("\n");
}

void delete(struct Node *t){
  if(t==n) return;
  t->pre->next = t->next;
  t->next->pre = t->pre;
}

void delete2(){
  delete(n->next);
}

void delete3(){
  delete(n->pre);
}

void deletekey(int key){
  delete(lserch(key));
}

void insert(int key){
  struct Node *x = (struct Node *)malloc(sizeof(struct Node));
  x->key=key;
  x->next=n->next;
  n->next->pre=x;
  n->next=x;
  x->pre=n;
}

int main(int argc, char const *argv[]) {
  int key,n,i;
  int size=0;
  char s[20];
  int np=0,nd=0;
  scanf("%d",&n);
  init();
  for (i = 0; i < n; i++) {
    scanf("%s%d",s,&key);
    if(s[0]=='i'){
      insert(key);
      np++;
      size++;
    }else if(s[0]=='d'){
      if(strlen(s)>6){
        if(s[6]=='F') delete2();
        else if(s[6]=='L')delete3();
      }else{
        deletekey(key);
        nd++;
      }
      size--;
    }
  }
  list();
  return 0;
}