如何使链表在 C++ 中有序？

Question

#include <iostream>
#include <string>
using namespace std;
struct node
{
    int num;
    node*next;
};

bool isEmpty(node *head);
char menu();
void insertasfirstelement(node *&head, node *&last, int num);
void insert(node *&head, node *&last, int num);
void remove(node *&head, node *&last);
void showlist(node*c);

bool isEmpty(node*head)
{
    if(head == NULL)
        return true;
    else
        return false;
}
char menu()
{
    char choice;

    cout << "\n\nMenu:\n";
    cout << "\n1. Add an item";
    cout << "\n2. Remove an item";
    cout << "\n3. Show the list";
    cout << "\n4. Exit" <<endl;

    cin >> choice;
    return choice;
}
void insertasfirstelement(node *&head, node*&last, int num)
{
    node * temp = new node;
    temp ->num=num;
    temp ->next=NULL;
    head = temp;
    last = temp;
}
void insert(node *&head, node *&last, int num)
{
    if(isEmpty(head))
        insertasfirstelement(head,last,num);
    else
    {
        node *temp = new node;
        temp ->num=num;
        temp ->next=NULL;
        last ->next= temp;
        last = temp;
    }
}
void remove(node *&head, node *&last)
{
    if(isEmpty(head))
        cout << "List is empty\n";
    else if(head ==  last)
    {
        delete head;
        head = NULL;
        last = NULL;
    }
    else
    {
        node *temp = head;
        head = head -> next;
        delete temp;

    }
}
void showlist(node*c)
{
    if(isEmpty(c))
        cout <<"The list is empty\n";
    else
    {
        cout << "The values are: \n";
        while(c !=NULL)
        {
            cout << c -> num << endl;
            c = c -> next;
        }
    }
}
int main()
{
    node *head=NULL;
    node *last=NULL;
    char choice;
    int num;

    do{
        choice = menu();

        switch(choice)
        {
            case '1':   cout << "Please enter a number: ";
                        cin >> num;
                        insert(head, last, num);
                        break;
            case '2':   remove(head,last);
                        break;
            case '3':   showlist(head);``
                        break;
            default:    cout << "System exit\n";
        }
    } while(choice != '4');
}

So I have been able to get a working linked list going. But I haven't been able to figure out how to get it in order. I also don't know how to make it so I can delete a number that I have inserted. I'm trying to understand how linked lists work better so if you can provide some help it would be greatly appreciated.

Answer 1

You can make linked list ordered by comparing the data field of the node

Try this way :

1 - Till the node is NULL travel through linked list.

2 - If data of the current node is less than the data of the next node then swap the both data.

3 - else move to the next node

And if you want to order linked list without swapping data ... ie changing the next of node then you can also do that way..

For deleting the node

try this way

1- Ask user to enter data of the node which he/she wants to delete.

2- search for that node which has that data.

3- if that node is found delete that node.

4- else find node in remaining linked list.

Note : This is just like how to do... you have to build the logic so that you can maintain consistency in the linked list :)

Answer 2

Both of the following functions assume you want a linked list sorted in ascending order.

To insert a new node into your linked list object, you could try:

void insert(node *&head, node *&last, int num)
{
    if(isEmpty(head))         
        insertasfirstelement(head,last,num);
    else if(num > tail->num)    //if new number to be inserted is greater than tail, insert at end and avoid needless iteration through list
    {
        node* temp = new node;
        temp->num = num;
        temp-next = NULL;
        last->next = temp;
        last = temp;
    }
    else
    {
        node *temp = new node;
        temp ->num=num;
        temp ->next=NULL;
        node* ptr = head;    //to lead iteration through linked list
        node* prev = NULL;  //to trail ptr
        while(ptr && (ptr->num > temp->num))  //iterate through linked list while new node's value is less than ptr's value and ptr is not beyond end of list (assuming last->next == NULL)
        {
            prev = ptr;
            ptr = ptr->next;
        }
        if(!prev)            // if prev never iterated, the temp node is the new head node
       {
           head = temp;
           head->next = ptr;
       }
       else if(prev == last)  //if prev is tail, the temp node is the new tail node
       {
           prev->next = temp;
           tail = temp;
       }
       else   //otherwise, insert temp node in between prev and ptr
       {
            prev-next = temp;
            temp->next = ptr;
       }
    }
}

To delete a node from your linked list object try:

void remove(node *&head, node *&last, int x) // added a int parameter to determine which node is to be removed
{
    if(isEmpty(head))
        cout << "List is empty\n";
    else 
    {
        node* ptr = head;
        node* prev = NULL;
        while(ptr && (ptr->num != x)   //iterate through linked list while desired value to be deleted is not found and have not gone through entire list
        {
            prev = ptr;
            prt = ptr->next;
        } 
        if(!prev)      //if prev never iterated, the head is the value to be deleted
        {
            node* garbage = head;
            head = head->next;
            delete garbage;
        }
        else if(ptr==tail)   //if the value to be deleted is the tail's value, reassign tail to prev and delete ptr
        {
           tail = prev;
           tail->next = NULL;
           delete ptr;
        }
        else if(!ptr)   //if ptr == NULL, value to be deleted was not in list
        {
            cout << "Value " << x << " could not be found in list.\n";     // or throw exception 
        }
        else      //reassign prev's next to ptr's next and delete ptr;
        {
            prev-next = ptr->next;
            delete ptr;
        }
    }
}