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>

struct node{
    unsigned int key;
    struct node *next;
    struct node *prev;
};

typedef struct node * NodePointer;

NodePointer nil;

void init();
void insert(int);
void deleteNode();
void deleteFirst();
void deleteLast();
void delete(int);
NodePointer listSerach(int);
void printList();

int main(){
    int key, n, a;
    int size=0;
    char command[20];
    
    scanf("%d",&n);
    init();
    //printf("n = %d\n",n);
    for(a = 0; a < n; a++){
        scanf("%s",command);
        //printf("%s",command);
        if(command[0] == 'i'){
            scanf("%d",&key);
            insert(key);
        }
        else if(command[0] == 'd'){
            if(strlen(command) > 6){
                if(command[6] == 'F') deleteFirst();
                else if(command[6] == 'L') deleteLast();
            }
            else{
                scanf("%d",&key);
                delete(key);
            }
        }
        //printList();
    }
    printList();
    return 0;
}

void init(){
    nil = malloc(sizeof(struct node));
    nil->next = nil;
    nil->prev = nil;
}

void insert(int key){
    NodePointer x;
    x = malloc(sizeof(struct node));
    x->key = key;
    x->next = nil->next;
    x->prev = nil;
    nil->next->prev = x;
    nil->next = x;
}

void deleteNode(NodePointer x){
    x->prev->next = x->next; //x = nil->next
    x->next->prev = x->prev;
    free(x);
}

void deleteFirst(){
    NodePointer x;
    x = nil->next;
    if(x == nil) return;
    deleteNode(x);
}

void deleteLast(){
    NodePointer x;
    x = nil->prev;
    if(x == nil) return;
    deleteNode(x);
}

void delete(int key){
    NodePointer x;
    if((x = listSerach(key)) == nil) return;
    deleteNode(x);
}

NodePointer listSerach(int key){
    NodePointer current_node;
    current_node = nil->next;
    while(current_node != nil){
        if(current_node->key == key) return current_node;
        current_node = current_node->next;
    }
    return nil;
}


void printList(){
    NodePointer current_node;
    current_node = nil->next;
    int isfirst = 1;
    while(1){
        if(current_node == nil) break;
        if(isfirst == 0) printf(" ");
        printf("%d",current_node->key);
        current_node = current_node->next;
        isfirst = 0;
    }
    printf("\n");
}