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 *prev, *next;
}Node;

  Node *nil;

void init(){ // ???????????????
    nil = (Node*)malloc(sizeof(Node));
    nil->next = nil;
    nil->prev = nil;
}

//??\???????????????????????¢??°

void insert(int key){  
    Node *x = (Node*)malloc(sizeof(Node));
    x->key = key;
    x->next = nil->next;
    nil->next->prev = x;
    nil->next = x;
    x->prev = nil;
}

Node* Search(int key){
    Node* x = nil->next;
    while(x != nil && x->key != key){
        x = x->next;
    }
    return x;
}

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

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

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

void deleteKey(int key){
    deleteNode(Search(key));
}

void print(){  //???????????????
    Node* x = nil->next;
    int i = 0;
    while(1){
        if(x == nil) break;
        if(i++ > 0) printf(" ");
        printf("%d", x->key);
        x = x->next;
    }
    printf("\n");
}

int main(void){
    int n,key,i,size = 0;
    char com[20];
    scanf("%d",&n);
    init();
    for(i=0;i<n;i++){
        scanf("%s %d", com, &key);
        if(com[0] == 'i'){	//insert?????????????????´???
            insert(key);
        }else if(com[0] == 'd'){	//delete?????????????????´??? ???????????????????????§?????\
        	if(strlen(com)>6){
        		if(strlen(com)>10){	//???????????????10????????\???->deleteFirst
        		deleteFirst();
        		}else if(strlen(com)>9){
        		deleteLast();
        		}
        }else{
            deleteKey(key);
        }
        }
    }
    print();
    return 0;
}