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: Wrong Answer)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Stack<int> st = new Stack<int>();

        int c = int.Parse(Console.ReadLine());
        string[] s;
        int n = 0;
        string ret = "";

        for (int i = 0; i < c; i++)
        {
            s = Console.ReadLine().Split(' ');
            if (s.Length != 1) n = int.Parse(s[1]);

            if (s[0] == "insert")
            {
                st.Push(n);
            }
            else if (s[0] == "delete")
            {
                if (st.Count != 0) 
                    st = DeleteStack(n, st);
            }
            else if (s[0] == "deleteFirst")
            {
                if (st.Count != 0) 
                    st.Pop();
            }
            else if (s[0] == "deleteLast")
            {
                if (st.Count != 0) 
                    st = DeleteStackLast(st);
            }

            ret = "";

            foreach (int a in st)
                ret = ret + a + " ";
            Console.WriteLine(ret.TrimEnd());
        }

        ret = "";

        while (0 < st.Count)
           ret = ret + st.Pop() + " ";
        Console.WriteLine(ret.TrimEnd());
    }

    static Stack<int> DeleteStack(int idx, Stack<int> st)
    {
        Stack<int> w = new Stack<int>();
        int n = 0;
        bool df = false;

        while (0 < st.Count)
        {
            n = st.Pop();

            if (!df && n == idx)
            {
                while (0 < w.Count)
                    st.Push(w.Pop());
                
                df = true;
                break;
            }

            w.Push(n);
        }

        if (!df)
        {
            while (0 < w.Count)
                st.Push(w.Pop());
        }

        return st;
    }

    static Stack<int> DeleteStackLast(Stack<int> st)
    {
        if (st.Count == 1)
        {
            st.Pop();
            return st;
        }

        Stack<int> w = new Stack<int>();
        int[] wk = st.ToArray();
        Array.Reverse(wk);
        
        for (int i = 1; i < wk.Length; i++)
            w.Push(wk[i]);

        return w;
    }
}