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 <ctype.h>
#include <string.h>

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

PNODE root;

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

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

void deleteNode(PNODE node) {
  if (node == root) {
    return;
  }
  node->next->prev = node->prev;
  node->prev->next = node->next;
  free(node);
}

void delete(key) {
  PNODE cur = root->next;
  while (cur != root) {
    if (cur->key == key) {
      deleteNode(cur);
      return;
    }
    cur = cur->next;
  }
}

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

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

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

void release(void) {
  PNODE tmp, cur = root->next;
  while (cur != root) {
    tmp = cur;
    cur = cur->next;
    deleteNode(tmp);
  }
  free(root);
} 

int main(void) {
  char cmd[100];
  int n, i, f;
  int key;

  init(); 

  scanf("%d", &n);

  f = 0;
  while (scanf("%s", cmd) > 0) {
    if (cmd[0] == 'i') {
      f = 1;
    } else if (cmd[0] == 'd') {
      switch (cmd[6]) {
      case '\0':
	f = 2;
	break;
      case 'F':
	deleteFirst();
	break;
      case 'L':
	deleteLast();
	break;
      }
    } else {
      key = atoi(cmd);
      if (f == 1) {
	insert(key);
      } else if (f == 2) {
	delete(key);
      }
    }
  }

  printNodes();

  release();

  return 0;
}