我无法在c++中显示链表

struct Node {
    int data;
    Node* next;
};

Node* head = nullptr; // 初始化头节点为空

void printList(Node* head) {
    Node* temp = head;
    while (temp != nullptr) {
        std::cout << temp->data << " ";
        temp = temp->next;
    }
    std::cout << std::endl;
}

Node* newNode = new Node();
if (!newNode) {
    std::cerr << "Memory allocation failed." << std::endl;
    return;
}

void addNode(int data) {
    Node* newNode = new Node();
    newNode->data = data;
    newNode->next = head;
    head = newNode; // 将新节点设置为头节点
}

#include <iostream>

struct Node {
    int data;
    Node* next;
};

void printList(Node* head) {
    Node* temp = head;
    while (temp != nullptr) {
        std::cout << temp->data << " ";
        temp = temp->next;
    }
    std::cout << std::endl;
}

void addNode(Node*& head, int data) {
    Node* newNode = new Node();
    newNode->data = data;
    newNode->next = head;
    head = newNode;
}

int main() {
    Node* head = nullptr;
    addNode(head, 3);
    addNode(head, 2);
    addNode(head, 1);

    printList(head); // 应该输出: 1 2 3

    return 0;
}