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: Accepted)

function DoublyLink(value) {
    this.value = value;
    this.prev = null;
    this.next = null;
}

function DoublyLinkedList() {
    this.first = null;
    this.last = null;
}

DoublyLinkedList.prototype.insert = function(link) {
    if (this.first) {
        this.first.prev = link;
        link.next = this.first;
        this.first = link;
        return;
    }
    this.first = link;
    this.last = link;
};

DoublyLinkedList.prototype.delete = function(link) {
    var l = this.find(link);
    if (!l) {
        return;
    }
    var prev = l.prev;
    var next = l.next;

    if (l.prev) {
        l.prev.next = l.next;
        if (!l.next) {
            this.last = l.prev;
        }
    }
    if (l.next) {
        l.next.prev = l.prev;
        if (!l.prev) {
            this.first = l.next;
        }
    }
};

DoublyLinkedList.prototype.deleteFirst = function() {
    if (!this.first) {
        return;
    }
    if (!this.first.next) {
        this.first = null;
        this.last = null;
        return;
    }
    this.first.next.prev = null;
    this.first = this.first.next;
}

DoublyLinkedList.prototype.deleteLast = function() {
    if (!this.last) {
        return;
    }
    if (!this.last.prev) {
        this.first = null;
        this.last = null;
        return;
    }
    this.last.prev.next = null;
    this.last = this.last.prev;
}

DoublyLinkedList.prototype.find = function(link) {
    var l = this.first;
    while (l) {
        if (l.value === link.value) {
            return l;
        }
        l = l.next;
    }
    return null;
};

DoublyLinkedList.prototype.list = function() {
    var a = [];
    var l = this.first;
    while (l) {
        a.push(l);
        l = l.next;
    }
    return a;
};

(function(input) {
    var p = input.split('\n');
    var n = Number(p.shift());
    var cs = p.slice(0, n);
    var l = new DoublyLinkedList();
    cs.forEach(function(e) {
        l = command(l, e);
    });
    //console.log(l.list());
    console.log(l.list().map(function(e) {
        return e.value;
    }).join(' '));
})(require('fs').readFileSync('/dev/stdin', 'utf8'));

function command(list, c) {
    var a = c.split(' ');
    var com = a.shift();
    var arg = Number(a.shift());
    switch (com) {
        case 'insert':
            var l = new DoublyLink(arg);
            list.insert(l);
            break;
        case 'delete':
            var l = new DoublyLink(arg);
            list.delete(l);
            break;
        case 'deleteFirst':
            list.deleteFirst();
            break;
        case 'deleteLast':
            list.deleteLast();
            break;
    }
    return list;
}