nullptr = Node is not assigning correctly - c++

Question

please don't dig into me too hard, I am still steadily learning and ran into an issue when trying to construct an AVL tree. When iterating through the tree on insert, I go until I reach a nullptr, create a new node, and assign that ptr to the nullptr. The value is never accepted though. Can someone find the error and explain it to me? ty!
#ifndef AVLTree_hpp
#define AVLTree_hpp
#include <stdio.h>
#include <stack>
template<typename T>
class AVLTree{
private:
struct Node{
T val;
Node* left;
Node* right;
int height;
Node(T V)
:left{nullptr},right{nullptr}
{
val = V;
}
~Node(){
}
};
Node* head;
void rightRotate(Node*& node);
void leftRotate(Node*& node);
void leftRight(Node*& node);
void rightLeft(Node*& node);
public:
AVLTree();
~AVLTree();
AVLTree(const AVLTree &c);
AVLTree(AVLTree &&c);
AVLTree &operator=(const AVLTree &c);
AVLTree &operator=(AVLTree &&c);
void add(T value);
int getHeight(Node* n);
};
template <typename T>
AVLTree<T>::AVLTree()
:head{nullptr}{
}
template <typename T>
AVLTree<T>::~AVLTree(){
}
template <typename T>
void AVLTree<T>::rightRotate(Node*& node){
Node* temp = node;
node = node->left;
Node* leftLL = node->right;
temp->left = leftLL;
node->right = temp;
}
template <typename T>
void AVLTree<T>::leftRotate(Node*& node) {
Node* temp = node;
node = node->right;
Node* yL = node->left;
temp->right = yL;
node->left = temp;
}
//left right condition
template <typename T>
void AVLTree<T>::leftRight(Node*& node) {
leftRotate(node->left);
rightRotate(node);
}
//right left condition
template <typename T>
void AVLTree<T>::rightLeft(Node*& node){
rightRotate(node->right);
leftRotate(node);
}
template <typename T>
void AVLTree<T>::add(T value){
if(head==nullptr){
head = new Node(value);
return;
}
std::stack<Node*> st;
Node* it = head;
while(it!=nullptr){
st.push(it);
if(value <= it->val){
it = it->left;
}else{
it=it->right;
}
}
//here is where the it is not assigned to the new node pointer.
//I have tested it and the node is created, "it" just does not hold the value at any point.
it = new Node(value);
int count = 0;
while(!st.empty()){
int balance = getHeight(st.top()->left) - getHeight(st.top()->right);
if(balance > 1){
if(st.top()->left!= nullptr&&st.top()->left!=nullptr){
leftRotate(st.top());
}else{
leftRight(st.top());
}
}else if(balance<-1){
if(st.top()->right!=nullptr&&st.top()->right!=nullptr){
rightRotate(st.top());
}else{
rightLeft(st.top());
}
}
st.pop();
if(++count==4){
break;
}
}
}
template <typename T>
int AVLTree<T>::getHeight(Node* n){
int max =0;
if(n!=nullptr){
max = std::max(getHeight(n->left),getHeight(n->right))+1;
}
return max;
}
#endif /* AVLTree_hpp */

Answer 1

It is a copy of the pointer and updating it has no effect on the original pointer. You need to do something like this instead:
Node* it = head;
bool left = true;
while(it!=nullptr){
st.push(it);
left = value <= it->val;
if(left){
it = it->left;
}else{
it=it->right;
}
}
it = new Node(value);
if (left){
stack.top()->left = it;
} else {
stack.top()->right = it;
}

Answer 2

Consider this simplified version of your code:
#include <iostream>
struct Linked {
Linked* next;
};
int main(int argc, char** argv) {
Linked l0 {nullptr};
// Case 1: Does not work
std::cout << "case 1" << std::endl;
Linked* node = l0.next;
node = new Linked {nullptr};
std::cout << "node=" << std::hex << node << std::endl;
std::cout << "l0.next=" << std::hex << l0.next << std::endl;
free(node);
std::cout << std::endl;
// Case 2: Works
std::cout << "case 2" << std::endl;
l0.next = new Linked {nullptr};
std::cout << "l0.next=" << std::hex << l0.next << std::endl;
free(l0.next);
l0.next = nullptr;
std::cout << std::endl;
// Case 3: Works
std::cout << "case 3" << std::endl;
Linked** nodeP = &(l0.next);
*nodeP = new Linked {nullptr};
std::cout << "*nodeP=" << std::hex << *nodeP << std::endl;
std::cout << "l0.next=" << std::hex << l0.next << std::endl;
free(l0.next);
l0.next = nullptr;
}
Which outputs:
$ ./main
case 1
node=0x7fba0d400620
l0.next=0x0
case 2
l0.next=0x7fba0d400620
case 3
*nodeP=0x7fba0d400620
l0.next=0x7fba0d400620
Case 1: does not work because the new Node is assigned to a copy of the left/right child pointer (i.e. not the actual child node from the parent node)
Case 2: works as expected because the new node is assigned directly to one of the parent's child node.
Case 3: also works because instead of assigning the new node to a copy of the child pointer, you assign it to a pointer referencing the pointer to child itself. In this respect, case 2 and 3 are equivalent.