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>

typedef struct _node {
  int key;
  struct _node *next, *prev;
} NODE, *PHODE;
PHODE root;

void init() {
  root = (NODE *)malloc(sizeof(NODE));
  root->next = root;
  root->prev = root;
}

void insert(int key) {
  PHODE t = (NODE *)malloc(sizeof(NODE));
  t->key = key;
  t->next = root->next;
  t->prev = root;
  root->next->prev = t;
  root->next = t;
}

void deleteNode(NODE *cur) {
  cur->next->prev = cur->prev;
  cur->prev->next = cur->next;
  free(cur);
}

void _delete(int key) {
  PHODE cur = root->next;
  while (cur != root) {
    if (cur->key  == key) {
      deleteNode(cur);
      break;
    }
    else {
      cur = cur->next;
    }
  }
}

void deleteFirst() {
  if (root != root->next)
    deleteNode(root->next);
}

void deleteLast() {
  if (root != root->prev)
    deleteNode(root->prev);
}

void printNodes() {
  PHODE cur = root->next;
  while (1) {
    printf("%d", cur->key);
    if ((cur = cur->next) == root) {
      printf("\n");
      break;
    }
    else {
      printf(" ");
    }
  }
}

int main() {
  init();
  int n, f, key;
  char command[255];
  scanf("%d", &n);
  while (n--) {
    f = 0;
    scanf("%s", command);
    if ('i' == command[0]) {
      scanf("%d", &key);
      insert(key);
    }
    else {
      if ('F' == command[6]) {
        deleteFirst();
      } else if ('L' == command[6]) {
        deleteLast();
      } else {
        scanf("%d", &key);
        _delete(key);
      }
    }
  }
  printNodes();
  return 0;
}