206.java

/**
 * 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) {
        ListNode pre = null;
        ListNode curr = head;
        ListNode next = null;

        // 三指针依次向后推进

        while(curr!=null){

            next = curr.next;
            
            curr.next=pre;
            pre = curr;
            curr = next;
            

        }
        return pre;
        

    }
}

// 灵神的版本，本质在于交换
/**
 * 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) {
        ListNode pre = null;
        ListNode cur = head;
        while(cur!=null){
            // 提前把下一个节点存好了
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}

// 第三次递归方法
/**
 * 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||head.next==null){
            return head;
        }
        // 保存我的下一个，让他继续递归
        ListNode newHead = reverseList(head.next);
        // 改变我
        head.next.next = head;
        head.next = null;
        // 返回头指针
        return newHead;

    }
}