35. 翻转链表

描述

翻转一个链表

样例

给出一个链表1->2->3->null，这个翻转后的链表为3->2->1->null

挑战

在原地一次翻转完成

代码

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

1. 两根指针

public class Solution {
    /*
     * @param head: n
     * @return: The new head of reversed linked list.
     */
    public ListNode reverse(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        
        ListNode prev = null;
        while (head != null) {
            ListNode temp = head.next;
            head.next = prev;
            // 两行的赋值顺序别写反了
            prev = head;
            head = temp;
        }
        
        return prev;
    }
}

2. 用递归

public class Solution {
    /*
     * @param head: n
     * @return: The new head of reversed linked list.
     */
    public ListNode reverse(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        
        // 从 head.next 开始所有链表全部被翻转
        // 但不包括 head.next 的前向指针
        ListNode p = reverse(head.next);
        head.next.next = head;
        head.next = null;
        
        // 返回链表头结点
        return p;
    }
}