Faulty avltree implementation c++ - c++

Question

this is my current avltree implementation, avltree.h:
#pragma once
#include <math.h>
#include <algorithm>
#include <iostream>
namespace avltree {
template <class T>
struct node {
static node * null_node;
node *left = null_node;
node *right = null_node;
node *parent = null_node;
int height = 0;
T value;
node(T value) :value(value) {}
node(T value, int height) :height(height), value(value) {}
};
template <class T>
node<T>* node<T>::null_node = new node(0, -1);
template <class T>
struct avltree {
public:
node<T> *root;
avltree(T value);
node<T> *insert(T value);
void print(void);
void print_with_height(void);
private:
node<T> *insert(T value, node<T> *x);
node<T> *left_rotate(node<T> *x);
node<T> *right_rotate(node<T> *x);
void retrace(node<T> *n);
void update_root();
void print(node<T> *n);
void print(node<T> *n, int depth);
void update_height(node<T> *n);
};
template <class T>
avltree<T>::avltree(T value) :root(new node<T>(value)) { }
template <class T>
node<T> *avltree<T>::insert(T value) {
auto n = insert(value, root);
update_root();
return n;
}
template <class T>
void avltree<T>::retrace(node<T> *n) {
while (n != node<T>::null_node) {
update_height(n);
if (n->left->height - n->right->height > 1) {
if (n->left->left->height >= n->left->right->height) {
right_rotate(n);
}
else {
left_rotate(n);
right_rotate(n);
}
}
else
if (n->right->height - n->left->height > 1) {
if (n->right->right->height >= n->right->left->height) {
left_rotate(n);
}
else {
right_rotate(n);
left_rotate(n);
}
}
n = n->parent;
}
}
template <class T>
node<T> *avltree<T>::insert(T value, node<T> *n) {
if (n->value > value) {
if (n->left != node<T>::null_node) {
n->left->height++;
insert(value, n->left);
}
else {
auto new_node = new node<T>(value);
n->left = new_node;
new_node->parent = n;
retrace(n);
return new_node;
}
}
if (n->value < value) {
if (n->right != node<T>::null_node) {
n->right->height++;
insert(value, n->right);
}
else {
auto new_node = new node<T>(value);
n->right = new_node;
new_node->parent = n;
retrace(n);
return new_node;
}
}
update_height(n);
return n;
}
template <class T>
node<T> *avltree<T>::left_rotate(node<T> *x) {
node<T> *y = x->right;
if (y == node<T>::null_node) {
return node<T>::null_node;
}
y->parent = x->parent;
if (x->parent->right == x) {
x->parent->right = y;
}
else {
x->parent->left = y;
}
y->left = x;
x->parent = y;
x->right = y->left;
x->right->parent = x;
update_height(x);
update_height(y);
return x;
}
template <class T>
node<T> *avltree<T>::right_rotate(node<T> *x) {
node<T> *y = x->left;
if (y == node<T>::null_node) {
return node<T>::null_node;
}
y->parent = x->parent;
if (x->parent->right == x) {
x->parent->right = y;
}
else {
x->parent->left = y;
}
y->right = x;
x->parent = y;
x->left = y->right;
x->left->parent = x;
update_height(x);
update_height(y);
return x;
}
template <class T>
void avltree<T>::update_root() {
auto n = root, last = root;
while (n != node<T>::null_node) {
last = n;
n = n->parent;
}
root = last;
}
template <class T>
void avltree<T>::print(void) {
print(root);
cout << endl;
}
template <class T>
void avltree<T>::print(node<T> *n) {
if (n->left != node<T>::null_node) {
print(n->left);
}
cout << n->value << " ";
if (n->right != node<T>::null_node) {
print(n->right);
}
}
template <class T>
void avltree<T>::print(node<T> *n, int height) {
if (n->left != node<T>::null_node) {
print(n->left, height);
}
if (n->height == height) {
std::cout << n->value << ", ";
}
if (n->right != node<T>::null_node) {
print(n->right, height);
}
}
template <class T>
void avltree<T>::print_with_height(void) {
int height = root->height;
for (int height = root->height; height >= 0; height--) {
std::cout << "height = " << height << "\t:";
print(root, height);
std::cout << std::endl;
}
std::cout << std::endl;
}
template <class T>
void avltree<T>::update_height(node<T> *n) {
n->height = std::max(n->left->height, n->right->height) + 1;
}
}
And the following is main file:
#include "stdafx.h"
#include "avltree.h"
int main()
{
avltree::avltree<int> tree(1);
for (int i = 2; i < 128; i++) {
tree.insert(i);
}
tree.print_with_height();
return 0;
}
The problem with above code is that at iteration 124 it will generate an infinite loop inside retrace call in insert. Basically, the nodes will keep being rotated in such a way that root node cannot be reached (resulting in an infinite loop). I have checked the trivial examples (for a few nodes) and they work. Everything for a run is provided, I would appreciate if someone with experience in avltrees could take their time and run this program, perhaps finding out what goes wrong inside retrace/rotations.

Answer 1

Getting an infinite loop like that indicates a likely problem with your rotate functions. A quick look at left_rotate shows
y->left = x;
x->parent = y;
x->right = y->left;
What is y->left in that last assignment? x, so what you're really is x->right = x. This is clearly a problem that needs to be fixed.
A similar problem exists in right_rotate.