Why is this if statement triggered in this C++ code? - c++

Question

This code is supposed to reverse a linked list. The following code returns an empty linked list even when provided with a non empty list.
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode* curr, *prev, *next;
if (head == NULL)
{
return head;
}
curr = head;
prev = NULL;
while (curr != NULL)
{
next = curr -> next;
curr -> next = prev;
prev = curr;
curr = next;
}
head = prev;
return head;
}
};
While this code strangely works where I added a cout statement just to check if the else was triggered.
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode* curr, *prev, *next;
if (head == NULL)
{
cout << "Triggered";
return head;
}
curr = head;
prev = NULL;
while (curr != NULL)
{
next = curr -> next;
curr -> next = prev;
prev = curr;
curr = next;
}
head = prev;
return head;
}
};
Can someone please explain why this is happening?

Answer 1

Pretty simple, you have to initialize the pointers, else it leads to unexpected behavior that includes not showing it at all or just showing it if an initialized cout is triggered - but it doesn't have to do anything and that's up to your compiler implementation.
//cpp17
listNode* curr{}, *prev{}, *next{};
//before
listNode* curr = nullptr, *prev = nullptr, *next = nullptr;
It is still not in the reverse order as you intended to do.
class Solution {
public:
ListNode* reverseList(ListNode* head) {
listNode* curr{}, *prev{}, *next{};
//ListNode* curr, *prev, *next;
if (head == NULL)
{
return head;
}
curr = head;
prev = NULL;
while (next != NULL)
{
next = curr -> next;
curr -> next = prev;
prev = curr;
curr = next;
}
head = prev;
return head;
}
};
cheers :)

Answer 2

Like mentioned before I found time to write a solution for an other approach of solving your problem to reverse a linked list via class. For a better understanding for beginners I skipped the rule of three/five and initialized the list in the main function and not via constructor in the class:
#include <iostream>
class listElement
{
std::string data;
listElement* next;
listElement* last;
public:
void setData(std::string);
void append(std::string);
void displayElements();
void reverseDisplayElements(listElement*);
void freeMemory();
listElement* reverseList(listElement*);
};
void listElement::setData(std::string newData)
{
last = this;
data = newData;
next = nullptr;
}
void listElement::append(std::string newData)
{
// Double linked list
// last->next = new listElement();
// last->next->data = newData;
// last->next->next = nullptr;
// last = last->next;
// Singly linked list
//has next the value nullptr?
//If yes, next pointer
if (next == nullptr)
{
next = new listElement();
next->data = newData;
next->next = nullptr;
}
//else the method again
else
next->append(newData);
}
listElement* listElement::reverseList(listElement* head)
{
//return if no element in list
if(head == nullptr)
return nullptr;
//initialize temp
listElement* temp{};
while(head != nullptr){
listElement* next = head->next;
head->next = temp;
temp = head;
head = next;
}
return temp;
}
void listElement::displayElements()
{
//cout the first entry
std::cout << data << std::endl;
//if the end is not reached, call method next again
if (next != nullptr)
next->displayElements();
}
void listElement::reverseDisplayElements(listElement*head)
{
//recursiv from the last to the list beginning - stop
listElement *temp = head;
if(temp != nullptr)
{
if(temp->next != nullptr)
{
reverseDisplayElements(temp->next);
}
std::cout << temp->data << std::endl;
}
}
void listElement::freeMemory()
{
//If the end is not reached, call the method again
if (next != nullptr)
{
next->freeMemory();
delete(next);
}
}
int main ()
{
//Pointer to the Beginning of the list
listElement* linkedList;
//Creating the first element
linkedList = new listElement();
//Write data in the first element
linkedList->setData("Element 1");
//add more elements
linkedList->append("Element 2");
linkedList->append("Element 3");
linkedList->append("Element 4");
//display list
linkedList->displayElements();
//space divider
std::cout << "\nPrint in reverse order:" << std::endl;
//display list in reverse order
//pass list beginning as stop point
linkedList->reverseDisplayElements(linkedList);
std::cout << std::endl;
linkedList->displayElements();
std::cout << "\nReverse elements:" << std::endl;
linkedList = linkedList->reverseList(linkedList);
linkedList->displayElements();
std::cout << std::endl;
//destruct the list and free memory
linkedList->freeMemory();
delete(linkedList);
return 0;
}
Btw. there are many different solutions for that task.