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>
#include <assert.h>

#define MAX_C 12

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

typedef struct node* NodePointer;
NodePointer head;

void init(void);
int getType(char*);
void insert(int);
void _delete(int);
void deleteFirst(void);
void deleteLast(void);
void deleteNode(NodePointer);
NodePointer search(int);
void print(void);

int main(){
  int N,i,key;
  char command[MAX_C];
  scanf("%d",&N); init();
  for(i = 0 ; i < N ; i++){
    scanf("%s",command);
    switch(getType(command)){
    case 0:
      scanf("%d",&key);
      insert(key);
      break;
    case 1:
      scanf("%d",&key);
      _delete(key);
      break;
    case 2:
      deleteFirst();
      break;
    case 3:
      deleteLast();
      break;
    default:
      assert(0);
    }
  }
  print();
  return 0;
}

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

int getType(char *command){
  if(!strcmp(command,"insert")){
    return 0;
  }else if(!strcmp(command,"delete")){
    return 1;
  }else if(!strcmp(command,"deleteFirst")){
    return 2;
  }else{
    return 3;
  }
  return -1;
}

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

void _delete(int key){
  NodePointer x = search(key);
  if(x == NULL){ return; }
  deleteNode(x);
}

void deleteFirst(){
  NodePointer x = head->next;
  if(x == head){ return; }
  deleteNode(x);
}

void deleteLast(){
  NodePointer x = head->prev;
  if(x == head){ return; }
  deleteNode(x);
}

void deleteNode(NodePointer x){
  NodePointer y = x->prev,z = x->next;
  y->next = z; z->prev = y;
  free(x);
}

NodePointer search(int key){
  NodePointer cur;
  for(cur = head->next ; cur != head ; cur=cur->next){
    if(cur->key == key){
      return cur;
    }
  }
  return NULL;
}

void print(){
  int i;
  NodePointer cur;
  for(i = 0,cur = head->next ; cur != head ; cur=cur->next,i++){
    if(i > 0){ printf(" "); }
    printf("%d",cur->key);
  }
  printf("\n");
}