struct ListNode {
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(nullptr) {}
};

ListNode* reverseList(ListNode* head) {
    ListNode* prev = nullptr;
    ListNode* curr = head;
    while (curr != nullptr) {
        ListNode* next = curr->next;  // 保存下一个节点
        curr->next = prev;            // 反转当前节点指针
        prev = curr;                  // 移动 prev 前进一步
        curr = next;                  // 移动 curr 前进一步
    }
    return prev;  // 新的头节点
}

ListNode* reverseListRecursive(ListNode* head) {
    if (head == nullptr || head->next == nullptr) {
        return head;
    }
    ListNode* newHead = reverseListRecursive(head->next);
    head->next->next = head;
    head->next = nullptr;
    return newHead;
}

#include <iostream>
using namespace std;
<p>// 打印链表
void printList(ListNode* head) {
while (head) {
cout << head->val << " ";
head = head->next;
}
cout << endl;
}</p><p>int main() {
// 创建链表 1->2->3->4
ListNode* head = new ListNode(1);
head->next = new ListNode(2);
head->next->next = new ListNode(3);
head->next->next->next = new ListNode(4);</p><pre class='brush:php;toolbar:false;'>cout << "原链表: ";
printList(head);

head = reverseList(head);  // 反转

cout << "反转后: ";
printList(head);

return 0;

}