How to make to linked list in C? - c

Question

Using two linked lists or something similar, I want to add "ABCD", then I want to get "ABDC".
I'm not sure if my code is right, I'm a noob in C.
The code should have pointer and malloc. I made it like this:
struct Node
{
char data;
struct Node* next;
};
struct Node* newNode(char data)
{
struct Node* node = (struct Node*)malloc(sizeof(struct Node));
node->data = data;
node->next = NULL;
return node;
}
struct Node* constructList()
{
struct Node* first = newNode(1);
struct Node* second = newNode(2);
struct Node* third = newNode(3);
struct Node* forth = newNode(4);
struct Node* head = first;
first->next = second;
second->next = third;
third->next=forth;
return head;
}
void printList(struct Node* head)
{
struct Node* ptr = head;
while (ptr)
{
printf("%d -> ", ptr->data);
ptr = ptr->next;
}
printf("NULL");
}

Answer 1

Your code is fine as shown, though it lacks error handling (and why are you using char to store integers?).
Do make sure you free() the nodes when you are done using them, eg:
void freeList(struct Node* head)
{
struct Node* ptr = head;
struct Node* next;
while (ptr)
{
next = ptr->next;
free(ptr);
ptr = next;
}
}
I would suggest changing constructList() to use a loop, eg:
struct Node* constructList()
{
struct Node *head = NULL;
struct Node **ptr = &head;
for (int value = 1; value <= 4; ++value)
{
*ptr = newNode(value);
if (*ptr == NULL)
{
freeList(head);
return NULL;
}
ptr = &((*ptr)->next);
}
return head;
}
You could then generalize constructList() to take an array of integers, or a string, as input and loop through that input adding a new node for each value. For example, try this:
struct Node
{
char data;
struct Node* next;
};
struct Node* newNode(char data)
{
struct Node* node = (struct Node*) malloc(sizeof(struct Node));
if (node)
{
node->data = data;
node->next = NULL;
}
return node;
}
void freeList(struct Node* head)
{
struct Node* ptr = head;
struct Node* next;
while (ptr)
{
next = ptr->next;
free(ptr);
ptr = next;
}
}
struct Node* constructList(const char *str)
{
struct Node *head = NULL;
struct Node **ptr = &head;
char ch;
if (str)
{
while ((ch = *str++) != '\0')
{
*ptr = newNode(ch);
if (*ptr == NULL)
{
freeList(head);
return NULL;
}
ptr = &((*ptr)->next);
}
}
return head;
}
void printList(struct Node* head)
{
struct Node* ptr = head;
while (ptr)
{
printf("%c -> ", ptr->data);
ptr = ptr->next;
}
printf("NULL");
}
struct Node *list = constructList("ABCD");
printList(list);
freeList(list);