迭代版实现

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null)
            return head;
        ListNode pre = head;
        ListNode cur = pre.next;
        while (cur != null) {
            ListNode pos = cur.next;
            cur.next = pre;
            pre = cur;
            cur = pos;
        }
        head.next = null;
        return pre;
    }
}

递归版实现

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null)
            return head;
        ListNode tail = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return tail;
    }
}