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<stdlib.h>
#include<string.h>

typedef struct node{
  int key;
  struct node *next, *pre;
} node;

node *primal;

node* search(int);
void init(void);
void insert(int);
void printlist(void);
void delnode(node *temp);
void delete_first(void);
void delete_last(void);
void delete_key(int);

int main(){
  int key, n ,i ,size = 0;
  char order[20];

  scanf("%d", &n);

  init();

  for(i = 0; i < n; i++){
    scanf("%s%d", order, &key);
    
    if(order[0] == 'i')
      insert(key);

    else if(order[0] == 'd'){
      if(strlen(order) > 6){
        if(order[6] == 'F')
          delete_first();
        else if(order[6] == 'L')
          delete_last();
      }
      else
        delete_key(key);
    }
  }

  printlist();
  
  return 0;
}

node* search(int key){
  node *temp;
  temp = primal -> next;

  while(temp != primal && temp -> key != key)
    temp = temp -> next;

  return temp;
}

void init(){
  primal = (node *)malloc(sizeof(node));
  primal -> next = primal;
  primal -> pre = primal;
}

void insert(int key){
  node *x;
  x = (node *)malloc(sizeof(node));
  x -> key = key;
  x -> next = primal -> next;
  x -> pre = primal;
  primal -> next -> pre = x;
  primal -> next = x;
  }
  
  void printlist(){
  node *temp = primal -> next;
  int i = 0;

  while(1){
    if(temp == primal)
      break;

    if(i != 0)
      printf(" ");

    printf("%d", temp -> key);
    temp = temp -> next;
    i++;
  }
  printf("\n");
}

void delnode(node *x){
  if(x == primal)
    return;

  x -> pre -> next = x -> next;
  x -> next -> pre = x -> pre;
  free(x);
}

void delete_first(){
  delnode(primal -> next);
}

void delete_last(){
  delnode(primal -> pre);
}

void delete_key(int key){
  delnode(search(key));
}