双指针如何用于链表实现？

Question

The following code works fine:

#include <iostream>
using namespace std;

struct Node{
    int data;
    struct Node *next;
    Node(int data, Node *next = nullptr){
        this->data = data;
        this->next = next;
    }
};

void push_back(Node **head, int data){
    if(*head == nullptr){
        *head = new Node(data);
    }
    else{
        Node *current = *head;
        while(current->next != nullptr){
            current = current->next;
        }
        current->next = new Node(data);
    }
}


void Print(Node **head){
    Node *current = *head;
    while(current != nullptr){
        cout << current->data << " ";
        current = current->next;
    }
    cout << endl;
}

int main(){
    Node *head = nullptr;
    push_back(&head, 5);
    push_back(&head, 2);
    push_back(&head, 1);
    push_back(&head, -7);
    Print(&head);

}

But when I try that bellow, nothing happens and head remains nullptr along with all the operations. All I did was that I passed single pointers to function instead of double pointers:

#include <iostream>
using namespace std;


struct Node{
    int data;
    struct Node *next;
    Node(int data, Node *next = nullptr){
        this->data = data;
        this->next = next;
    }
};

void push_back(Node *head, int data){
    if(head == nullptr){
        head = new Node(data);
    }
    else{
        Node *current = head;
        while(current->next != nullptr){
            current = current->next;
        }
        current->next = new Node(data);
    }
}


void Print(Node *head){
    Node *current = head;
    while(current != nullptr){
        cout << current->data << " ";
        current = current->next;
    }
    cout << endl;
}

int main(){
    Node *head = nullptr;
    push_back(head, 5);
    push_back(head, 2);
    push_back(head, 1);
    push_back(head, -7);
    Print(head);

}

I don't understand why do I need double pointers to make it work? Is the second program only sending a copy of head to the functions and nothing more?

Answer 1

void push_back(Node *head, int data){
    if(head == nullptr){
        head = new Node(data);
    }
    else{
        Node *current = head;
        while(current->next != nullptr){
            current = current->next;
        }
        current->next = new Node(data);
    }
}

you can't change the value of pointer head by push_back in main function. We can change a object by passing its pointer or reference to another function, but not by passing itself! So each time head = new Node(data); (try to change head ) actually not change head in the function which called push back() and caused memory overflow