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>

struct P{
  int key;
  struct P *prev,*next;
};
typedef struct P* node;
node F,L;

void insert(int x) {
  node z=F->next;
  node v=malloc(sizeof(struct P));
  v->key=x;
  F->next=v;
  v->prev=F;
  v->next=z;
  z->prev=v;
}

void delete(int x) {
  node z=F;
  while(z->key!=x && z!=L) z=z->next;
  node l=z->prev,r=z->next;
  l->next=r;
  r->prev=l;
}

void deleteF() {
  node z=F->next->next;
  F->next=z;
  z->prev=F;
}

void deleteL() {
  node z=L->prev->prev;
  L->prev=z;
  z->next=L;
}

int main() {
  int n;
  scanf("%d",&n);
  char c[10];
  int x;
  F=malloc(sizeof(struct P));
  L=malloc(sizeof(struct P));
  F->next=L;F->prev=F;
  L->prev=F;L->next=L;
  while(n--) {
    scanf("%s",c);
    if(c[0]=='i') {
      scanf("%d",&x);
      insert(x);
    } else if(c[6]=='F') deleteF();
    else if(c[6]=='L') deleteL();
    else {
      scanf("%d",&x);
      delete(x);
    }
  }
  node z=F->next;
  while(z!=L) {
    if(z!=F->next) printf(" ");
    printf("%d",z->key);
    z=z->next;
  }
  printf("\n");
  return 0;
}