image.png

解法

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null) {
            return head;
        }
        // 之前已经反转过的链表的位置
        ListNode pre = null;
        while (head != null) {
            // 维护后一个节点的位置
            ListNode next = head.next;
            // 当前节点的next指向之前已经反转好的链表
            head.next = pre;
            // pre指针后移
            pre = head;
            // 继续遍历
            head = next;
        }
        return pre;
    }
}