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++11 (judge verdict: Accepted)

#include <iostream>
using namespace  std;

struct StList
{
  int  m_nKey;
  StList *m_poPrev;
  StList *m_poNext;
  
  StList() :
    m_nKey(0), m_poPrev(nullptr), m_poNext(nullptr)
  { }

  StList(int nNum, StList &roHead) :
    m_nKey(nNum), m_poPrev(&roHead), m_poNext(roHead.m_poNext)
  { }

  ~StList() { }
};

void fnLinkedListSet(StList &roHead, StList &roTrail)
{
  char acCmd[80];
  int nMaxCmd, nNum;

  cin >> nMaxCmd;
  for (int i = 0; i < nMaxCmd; i++)
  {
    cin >> acCmd;
    if (acCmd[0] == 'i')
    {
      cin >> nNum;
      StList *poList = new StList(nNum, roHead);
      roHead.m_poNext->m_poPrev = poList;
      roHead.m_poNext = poList;
    }
    else if (acCmd[6] == 'F')
    {
      StList *poList = roHead.m_poNext;
      if (poList != &roTrail)
      {
        poList->m_poNext->m_poPrev = poList->m_poPrev;
        roHead.m_poNext          = poList->m_poNext;
        delete poList;
      }
    }
    else if (acCmd[6] == 'L')
    {
      StList *poList = roTrail.m_poPrev;
      if (poList != &roHead)
      {
        roTrail.m_poPrev         = poList->m_poPrev;
        poList->m_poPrev->m_poNext = poList->m_poNext;
        delete poList;
      }
    }
    else if (acCmd[0] == 'd')
    {
      cin >> nNum;
      StList *poList = roHead.m_poNext;
      while (poList != &roTrail)
      {
        if (poList->m_nKey == nNum)
        {
          poList->m_poNext->m_poPrev = poList->m_poPrev;     
          poList->m_poPrev->m_poNext = poList->m_poNext;
          delete poList;
          break;
        }
        poList = poList->m_poNext;
      }
    }
  }
}

void fnLinkedListGet(StList &roHead, StList &roTrail)
{
  StList *poList = roHead.m_poNext;
  while (poList != &roTrail)
  {
    if (poList != roHead.m_poNext) cout << " ";
    cout << poList->m_nKey;
    poList = poList->m_poNext;
    delete poList->m_poPrev;
  }
  cout << endl;
}

int main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);
 
  StList oHead, oTrail;
  oHead.m_poNext  = &oTrail;
  oTrail.m_poPrev = &oHead;
 
  fnLinkedListSet(oHead, oTrail);
  fnLinkedListGet(oHead, oTrail);

  return 0;
}