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 JavaScript (judge verdict: Wrong Answer)

const readline = require('readline');

class Node {
  constructor(val) {
    this.prev = null;
    this.next = null;
    this.val = val;
  }
}

class DoublyLinkedList {
  constructor() {
    this.first = null;
    this.last = null;
    this.size = 0;
  }

  insert(val) {
    let n = new Node(val);
    if (this.first) {
      let oldFirst = this.first;
      this.first = n;
      oldFirst.prev = this.first;
      this.first.next = oldFirst;
    } else {
      this.first = this.last = n;
    }
    this.size++;
  }

  delete(val) {
    for (let n = this.first; n !== null; n = n.next) {
      if (n.val === val) {
        if (n.prev === null) {
          this.deleteFirst();
        } else if (n.next === null) {
          this.deleteLast();
        } else {
          n.prev.next = n.next;
          n.next.prev = n.prev;
        }
        this.size--;
        break;
      }
    }
  }

  deleteFirst() {
    if (this.size === 0) {
      return;
    } else if (this.size === 1) {
      this.first = this.last = null;
      this.size = 0;
    } else {
      this.first = this.first.next;
      this.first.prev = null;
      this.size--;
    }
  }

  deleteLast() {
    if (this.size === 0) {
      return;
    } else if (this.size === 1) {
      this.first = this.last = null;
      this.size = 0;
    } else {
      this.last = this.last.prev;
      this.last.next = null;
      this.size--;
    }
  }
}

let rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

let dll = new DoublyLinkedList();

rl.on('line', function (input) {
  let arr = input.split(' ');
  let [cmd, param] = arr;
  if (dll[cmd]) {
    dll[cmd](param);
  }
})

rl.on('close', function () {
  let arr = [];
  for (let n = dll.first; n !== null; n = n.next) {
    arr.push(n.val);
  }
  console.log(arr.join(' '));
})