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;

NodePointer listSearch(int key){
    NodePointer t;
    for (t = nil->next; (t != nil) && (t->key != key); t = t->next);//fantastic
    return t;
}

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

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

void deleteNode(NodePointer t){
    /*if (t->next == nil){
	t->prev->next = nil;
	free(t);
    }else{
	t->next->prev = t->prev;
	t->prev->next = t->next;
	free(t);
    }
    return ;
    */
    if ( t == nil) return;
    t->prev->next = t->next;
    t->next->prev = t->prev;
    free(t);
}

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

void deleteLast(){
    deleteNode(nil->prev);
    //t = nil;
    //there are no data in the list
    //if (nil->next == nil) return ;
    //traverse the list to the end of the list
    //while(t->next != nil) t = t->next;
}

void delete(int key){
    NodePointer t;
    t = listSearch(key);
    if (t == nil) return ;
    else deleteNode(t);
    return ;
}

void insert(int key){
    NodePointer x;
    x = malloc(sizeof(struct node));
    x->key = key;
    //insert the new pointer into the head of the list
    x->next = nil->next;
    nil->next->prev = x;
    nil->next = x;
    x->prev = nil;
}
//------------------------------------------------------------
int main(){
    int key, n, i;
    int size = 0;
    char com[20];
    int np = 0, nd = 0;
    scanf("%d", &n);
    init();
    for ( i = 0; i < n; i++ ){
	scanf("%s%d", com, &key);
	if ( com[0] == 'i' ) {insert(key); np++; size++;}
	else if ( com[0] == 'd' ) {
	    if (strlen(com) > 6){
		if ( com[6] == 'F' ) deleteFirst();
		else if ( com[6] == 'L' ) deleteLast();
	    } else {
		delete(key); 
	    }
	    size--;
	}
    }

    printList();
    return 0;
}