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, *prev;
} Node;

Node *nil;

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

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

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

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

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

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

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

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;
}

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