将值按升序插入排序的链表中

Question

This function will asks the user to input an integer, then inserts it to the linked list in ascending order. Will not insert if current value already existed.

typedef struct _listnode{
    int item;
    struct _listnode *next;
} ListNode;         

typedef struct _linkedlist{
    int size;
    ListNode *head;
} LinkedList;           

void insertSortedLinkedList(LinkedList *l)
{
    ListNode *cur = l->head;
    ListNode* newNode = malloc(sizeof(ListNode)); // create the node to be inserted
    int x;
    printf("please input an integer you want to add to the linked list:");
    scanf("%d", &x);
    newNode->item = x;
    newNode->next = NULL;

    if (l->head == NULL) // linkedlist is empty, inserting as first element
    {
        l->head = malloc(sizeof(ListNode));
        l->head->item = x;
        l->head->next = NULL;
        l->size++;
    }
    else
    {

        if (x < l->head->item) // data is smaller than first element, we will insert at first element and update head.
        {
            newNode->next = l->head;
            l->head = newNode;
            l->size++;
            return;
        }
        while (cur->next != NULL) // loop through the linkedlist
        {
            if (cur->next->item > x) // next element is bigger than data, we will insert it now.
            {
                if (cur->item != x) // if current element is not same as data, it must not have already existed.
                {
                    newNode->next = cur->next;
                    cur->next = newNode;
                    l->size++;
                    return;
                }
            }
            if (cur->next == NULL) // we have reached the last element and data is even greater than that. we will then insert it as last element.
            {
                cur->next = newNode;
                l->size++;
                return;
            }
            cur = cur->next;
        }
    }
}

Somehow, there's a bug in it. When I try to insert the following, I get these result. It also doesn't insert if the data is bigger than those existed.

Insert : 10
Result : 10
Insert : 5
Result : 5 10
Insert : 8
Result : 5 8 10
Insert : 10
Result : 5 8 10
Insert : 7
Result : 5 7 8 10
Insert : 9
Result : 5 7 8 9 10
Insert : 6 
Result : 5 6 7 8 9 10
Insert : 5
Result : 5 6 5 7 8 9 10 << why?

Answer 1

You test for equality in the wrong place: you always skip the first node. You also need to improve the allocation scheme: you allocate memory twice for the head node and forget to free memory if the integer is already in the list.

Here is an improved version:

void insertSortedLinkedList(LinkedList *l)
{
    ListNode *cur, *newNode;
    int x;

    printf("please input an integer you want to add to the linked list:");
    if (scanf("%d", &x) != 1)
        return;

    newNode = malloc(sizeof(ListNode)); // create the node to be inserted
    newNode->item = x;
    newNode->next = NULL;

    if (l->head == NULL)
    {
        // linkedlist is empty, inserting as first element
        l->head = newNode;
        l->size++;
        return;
    }
    if (x < l->head->item)
    {
        // data is smaller than first element, we will insert at first element and update head.
        newNode->next = l->head;
        l->head = newNode;
        l->size++;
        return;
    }
    for (cur = l->head;; cur = cur->next) // loop through the linkedlist
    {
        if (cur->item == x)
        {
            // element already in the list
            free(newNode);
            return;
        }
        if (!cur->next || cur->next->item > x)
        {
            // next element is bigger than data or end of list, we will insert it now.
            newNode->next = cur->next;
            cur->next = newNode;
            l->size++;
            return;
        }
    }
}

This code can be made shorter using a pointer to the link:

void insertSortedLinkedList(LinkedList *l)
{
    ListNode **curp, *cur, *newNode;
    int x;

    printf("please input an integer you want to add to the linked list:");
    if (scanf("%d", &x) != 1)
        return;

    for (curp = &l->head; (cur = *curp) != NULL; curp = &cur->next) {
        if (cur->item == x)
            return;
        if (cur->item > x)
            break;
    }
    // cur element is bigger than data or end of list, we will insert it now.
    newNode = malloc(sizeof(ListNode)); // create the node to be inserted
    newNode->item = x;
    newNode->next = cur;
    *curp = newNode;
    l->size++;
}

Answer 2

Problem is that you never reach your if-condition because when cur->next == NULL your loop breaks:

while (cur->next != NULL) // loop through the linkedlist
    ....
        if (cur->next == NULL) // we have reached the last element and data is even greater than that. we will then insert it as last element.
        {
           ...
        }
    ...
    }

Instead, you should have used while(cur != NULL) as loop, and set cur->next == NULL as first if-condition, so that (cur->next->item doesn't crash your program when cur == NULL .

NOTE: in this case, your loop condition won't be important because return in if (cur->next == NULL) will be breaking the loop. It's only important not to quit loop before you get to execute that if -condition.