Singly Linked List Doesn't work when i am using Node(key,data) rather than Node.key and Node.data in C++

Question

Created a Node to use in Singly Linked List, Nodes are getting successful created by both methods ie n.key, n.data and n(data,key)

Code Works Properly when I use n.key, n.data method But when I use n(key, data) n1 got appended but n2 failed to do

#include <iostream>
using namespace std;
class Node{
    public:
        int key,data;
        Node* next;
        Node(){
            key = 0;
            data = 0;
            next = NULL;}
        Node(int k, int d){
            key = k;
            data = d;
            cout<<"Node created"<<endl;}};
class Singly_Linked_List{
    public:
        Node* head;
        Singly_Linked_List(){head = NULL;}
        Singly_Linked_List(Node* n){head = n;}
        Node* KeyExists(int k){
            Node* ptr = head;
            while(ptr!= NULL){
                if((ptr -> key) == k){return ptr;}
                ptr = ptr -> next;}
            return NULL;}
        void Append_Node(Node* n){
            if (head == NULL){head = n;
                cout<<"Node appended"<<endl;}
            else if(KeyExists(n->key)!=NULL){cout<<"Key already exists"<<endl;}
            else{Node* ptr = head;
                while(ptr -> next != NULL){ptr = ptr -> next;}
                ptr -> next = n;
                cout<<"Node appended"<<endl;}}};
int main(){
    /*
    Node n1,n2;
    n1.key = 1;
    n1.data = 10;
    n2.key = 2;
    n2.data = 20;
     */
    Node n1(1,10),n2(2,10);
    Singly_Linked_List s;
    s.Append_Node(&n1);
    s.Append_Node(&n2);
    return 0;
}

Answer 1

The constructor that takes the two arguments does not initialise next :

    Node(int k, int d){
        key = k;
        data = d;
        next = NULL;  // <--- was missing
    }