Linked List insertion isn't working in for/while loop

Question

I am learning DSA, and was trying to implement linked list but the insertion function that i wrote is not working in a for or while loop, its not the same when i call that function outside the loop, it works that way. I am not able to figure it out, please someone help me.

#include <iostream>

class Node {

public:
  int data;
  Node *next;

  Node(int &num) {
    this->data = num;
    next = NULL;
  }
};

class LinkedList {

  Node *head = NULL;

public:
  void insert(int num) {
    Node *tmp;
    if (head == NULL) {
      head = new Node(num);
      tmp = head;
    } else {
      tmp->next = new Node(num);
      tmp = tmp->next;
    }
  }

  void printList() {
    Node *tmp = head;
    while (tmp) {
      std::cout << tmp->data << "  ";
      tmp = tmp->next;
    }
    std::cout << std::endl;
  }

  void reverseList() {
    Node *curr = head, *prev = NULL, *nextNode;
    while (curr) {
      nextNode = curr->next;
      curr->next = prev;
      prev = curr;
      curr = nextNode;
    }
    head = prev;
  }
};

int main() {
  LinkedList list1;
  // This is not working
  int num;
  while (num != -1) {
    std::cin >> num;
    list1.insert(num);
  }

  // This is working
  // list1.insert(1);
  // list1.insert(2);
  // list1.insert(3);
  // list1.insert(4);
  // list1.insert(5);

  list1.printList();
  list1.reverseList();
  list1.printList();

  return 0;
}

I expect this after insertion

Edit: although @Roberto Montalti solved this for me, but before that I tried passing incrementing value using a for loop which worked but as soon as I pull that cin out it crashes. can someone tell me what's happening under the hood?

for (int i = 1; i <= 10; i++)
{
    list1.insert(i);
}

Answer 1

You need to loop until you reach the end of the list and then add the new node after that. Like this.

void insert(int num) {
    Node *tmp = head;
    if (head == NULL) {
        head = new Node(num);
    }
    else {
        while (tmp->next != NULL) {
            tmp = tmp->next;
        }
        tmp->next = new Node(num);
    }
  }

Answer 2

first of all you need to define a node for each of the tail and head of the list as follows

Node *h;
Node *t;

you may also separate the Node from the LinkedList class so you can modify easily

class Node{
public:
    int data;
    Node *next;
    Node(int data, Node* next);
    ~Node();
};
Node::Node(int data, Node* next)
{
    this->data= data;
    this->next= next;
}
Node::~Node(){}
}

after that you can try to add these functions to your LinkedList class so it can deal with other special cases such empty list or full, etc..

void addToHead(int data){
Node *x = new Node(data,h);
    h=x;
    if(t==NULL){
        t=x;
    }


void addToTail(int data){
Node *x = new Node(data,NULL);
    if(isEmpty()){
        h=t=x;
    }
    else
    {
        t->next=x;
        t=x;
    }
}

now for the insert function try this after you implemented the Node class and the other functions,

void insert(int v){
if(h==nullptr){addToHead(v); return;}
if(h->data>=v) {addToHead(v);return;}
if(t->data<=v) {addToTail(v); return;}
// In this case there is at least two nodes
Node *k=h->next;
Node *p=h;

while(k != nullptr){
if(k->data >v){
Node *z =new Node(v,k);
p->next=z;
return;
}
p=k;
k=k->next;
}

}

the idea of making all of this is not lose the pointer when it goes through elements in the Linked List so you don't end up with a run time error.

I hope this can be useful to you.

Answer 3

There was an issue with your insert function. Read about segmentation fault here https://www.geeksforgeeks.org/core-dump-segmentation-fault-c-cpp/#:~:text=Core%20Dump%2FSegmentation%20fault%20is,is%20known%20as%20core%20dump.

for a quick workaround you can use this

using namespace std;
#include <iostream>
    class Node
    {
    public:
        int data;
        Node *next;
        Node(int num)
        {
            this->data = num;
            next = NULL;
        }
    };
    
    class LinkedList
    {
    Node *head = NULL;
    
    public:
        void insert(int num)
        {
            Node *tmp= new Node(num);
            tmp->next=head;
            head=tmp;
        }
        void printList()
        {
            Node *tmp = head;
            while (tmp)
            {
                std::cout << tmp->data << "  ";
                tmp = tmp->next;
            }
            std::cout << std::endl;
        }
        void reverseList()
        {
            Node *curr = head, *prev = NULL, *nextNode;
            while (curr)
            {
                nextNode = curr->next;
                curr->next = prev;
                prev = curr;
                curr = nextNode;
            }
            head = prev;
        }
    };
    
    int main()
    {
        LinkedList list1;
        // This is not working
        int num,i=0,n;
        cout<<"Type the value of n";
        cin>>n;
        while (i<n)
        {
            cin >> num;
            cout<<num<<" "<<&num<<endl;
            list1.insert(num);
            i++;
        }

    
        list1.printList();
        list1.reverseList();
        list1.printList();
    
        return 0;
    }