Wie löscht man alternative Knoten in einer doppelt verknüpften Liste in C?

Question

Ich schrieb den folgenden Code, aber es funktioniert nicht mehr nach dem Ausführen der create() -Funktion. Ich möchte alternative Elemente löschen, die vom Hauptknoten ausgehen. Ist meine Funktion delete_Alt() korrekt? Bitte sag mir wo ich falsch liege.

#include <stdio.h> 
#include <stdlib.h> 

// using a structure 
typedef struct mynode { 
    int data; 
    struct mynode *prev; // to point to previous node 
    struct mynode *link; // to point to next node 
} node; 
node *head = NULL; 

// creating the list 
void create() { 
    node *p, *q; 
    int ch; 
    do { 
     p = (node *)malloc(sizeof(node)); 
     printf("enter data\n"); 
     scanf("%d", &p->data); 
     if (head == NULL) 
     { 
      p->prev = head; 
      q = p; 
     } 
     else 
     { 
      p->prev = q; 
      p->link = NULL; 
      q->link = p; 
      q = p; 
     } 
     printf("create another node?, press 1 "); 
     scanf ("%d",&ch); 
    } while(ch==1); 
} 

//to delete alternate elements 
void delete_Alt() { 
    if (head == NULL) 
     printf("Empty list...ERROR"); 

    node *previous, *current, *next;  
    previous = head; 
    current = head->link; 
    while (previous !=NULL && current != NULL) { 
     previous->prev = current->prev; 
     previous->link = current->link; 

     next = current->link; 
     previous->link = next; 
     next->prev = previous; 

     free(current); 
    } 
} 

// print the list 
void display() { 
    node *temp; 
    temp = head; 
    while (temp != NULL) { 
     printf("%d ",temp->data); 
     temp = temp->link; 
    } 
    printf("\n"); 
} 

int main() { 
    node *head = NULL; 
    create(); 
    printf("List before deleting is: "); 
    display(); 
    delete_Alt(); 
    printf("List after deleting is:  "); 
    display(); 
return 0; 
} 

Answer 1

In Ihrem Programm zugewiesen Sie einen Wert nur einmal den Kopf:

node *head = NULL; 

Dann seinen Wert nicht ändert.

Answer 2

Sie haben dem ersten erstellten Element in der Liste nie "head" zugewiesen. Daher ist es immer Null. Versuchen Sie folgendes:

if (head == NULL) 
{ 
    p->prev = head; 
    head = p;  
    q = p; 
} 

In Ihrem delete_alt, müssen Sie dies tun:

while (previous !=NULL && current != NULL) { 
    previous->link = current->link; 
    next = current->link; 
    free(current); 
    if(next) { 
     next->prev = previous; 
     current = next->link; 
    } 
    else current = NULL; 
    previous = next; 
} 

es versuchen hier: https://repl.it/HK2P/0

Answer 3

Sie erstellen sowie in Löschfunktion einige kleine Fehler machen ...

Hier ist der aktualisierte Code versuchen sie es ...

#include <stdio.h> 
#include <stdlib.h> 

// using a structure 
typedef struct mynode { 
    int data; 
    struct mynode *prev; // to point to previous node 
    struct mynode *link; // to point to next node 
} node; 
node *head = NULL; 

// creating the list 
void create() { 
    node *p, *q; 
    int ch; 
    do { 
     p = (node *)malloc(sizeof(node)); 
     printf("enter data\n"); 
     scanf("%d", &p->data); 
     p->link = NULL; 
     if (head == NULL) 
     { 
      p->prev = NULL; 
      head = p; 
     } 
     else 
     { 
      q = head; 
      while (q->link != NULL) 
      q = q->link; 
      p->prev = q; 
      q->link = p; 
     } 
     printf("create another node?, press 1 "); 
     scanf ("%d",&ch); 
    } while(ch==1); 
} 

//to delete alternate elements 
void delete_Alt() { 
    if (head == NULL) 
     printf("Empty list...ERROR"); 

    node *previous, *current, *next;  
    previous = head; 
    current = head->link; 
    while (previous !=NULL && current != NULL) 
    { 
     previous->link = current->link; 
     next = current->link; 
     free(current); 
     if(next) 
     { 
      next->prev = previous; 
      current = next->link; 
     } 
     else 
     current = NULL; 
     previous = next; 
    } 
} 

// print the list 
void display() { 
    node *temp; 
    temp = head; 
    while (temp != NULL) { 
     printf("%d ",temp->data); 
     temp = temp->link; 
    } 
    printf("\n"); 
} 

int main() { 
    node *head = NULL; 
    create(); 
    printf("List before deleting is: "); 
    display(); 
    delete_Alt(); 
    printf("List after deleting is:  "); 
    display(); 
return 0; 
}