How to add elements to the end of a linked list

问题

So I am trying to create a function in C++ that should add an element at the end of an linked list. This function that adds the elements should be called within the main-method. It should be possible to call it even if there is no element in the list.

What I have so far is the following:

int main()
{
   ListElement* l = new ListElement;
   l->digit = 9;
   l->next = NULL;
   appendList(l, 5);
   appendList(l, 7);

   printList(l);

   return 0;
}

void appendList(ListElement *&l, int newDigit)
{
    ListElement *lh = new ListElement;

    if(l == NULL)
    {
        l->digit = newDigit;
        l->next = NULL;
    }
    else
    {
        lh=l;

        while(lh != NULL)
        {
           lh=lh->next;
        }
        lh=lh->next;
        lh->digit = newDigit;
        lh->next = NULL;
        l = lh;
    }
}

Unfortunately it just isn't working. I've tried removing or adding parameters - nothing helped and I couldn't find an appropriate answer on the internet. So if anyone of you could help me, I'd be very, very happy because I'm kind of despairing here...

回答1:

Try the following

void appendList( ListElement * &head, int newDigit )
{
    if ( head == 0 )
    {
        head = new ListElement;

        head->digit = newDigit;
        head->next  = 0;
    }
    else
    {
        ListElement *next = head;

        while ( next->next ) next = next->next;

        next->next = new ListElement;
        next->next->digit = newDigit;
        next->next->next  = 0;
    }
} 


int main()
{
    ListElement *head = 0;

    appendList( head, 9 );
    appendList( head, 5 );
    appendList( head, 7 );

    printList( head );

    return 0;
}

回答2:

Look closely at:

if (l == NULL)
{
    l->digit = newDigit;
    l->next = NULL;
}

l == NULL .... l->digit You are dereferencing a NULL pointer!

Another problem is that you allocate lh in ListElement *lh = new ListElement; and then immediately overwrite its value with l in "else" block lh=l;.

Try something like this instead:

#include <cassert>
#include <iostream>

struct ListElement final {
    explicit ListElement(int digit) : digit{digit} {}

    int digit = 0;
    ListElement* next = nullptr;
};

void appendList(ListElement*& l, int newDigit);
void printList(ListElement* l);
void freeList(ListElement* l);

int main() {
    ListElement* l{nullptr};
    appendList(l, 9);
    appendList(l, 5);
    appendList(l, 7);
    printList(l);
    freeList(l);
}

void appendList(ListElement*& l, int newDigit) {
    if (!l) {
        // Creating first element of the list.
        l = new ListElement{newDigit};
        return;
    }

    // Since we got a single linked list and don't have a pointer
    // to its tail, iterate over the list to get the last element
    // to append to...
    auto tail = l;
    while (tail->next)
        tail = tail->next;
    assert(tail->next == nullptr);
    tail->next = new ListElement{newDigit};
}

void printList(ListElement* l) {
    unsigned int index = 0;
    while (l != nullptr) {
        std::cout << index++ << ": " << l->digit << '\n';
        l = l->next;
    }
}

void freeList(ListElement* l) {
    ListElement* tmp;
    while (l != nullptr) {
        tmp = l;
        l = l->next;
        delete tmp;
    }
}

来源：https://stackoverflow.com/questions/20384407/how-to-add-elements-to-the-end-of-a-linked-list