方法一：迭代法

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    public ListNode ReverseList (ListNode head) {
        // 两个指针分别保存要反转的节点和前一个节点
        ListNode prev = null, curr = head;
        while (curr != null) {
            ListNode next = curr.next;
            curr.next = prev;

            prev = curr;
            curr = next;
        }

        return prev; // prev 指向的节点就是新的头节点
    }
}

方法二：递归法

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    public ListNode ReverseList (ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }

        // 先反转后面的
        ListNode newHead = ReverseList(head.next);
        // 让后面的节点指向当前节点
        head.next.next = head;
        head.next = null; // 防止成环
        
        return newHead;
    }
}