LeetCode 206. Reverse Linked List（链表）

题目来源：https://leetcode.com/problems/reverse-linked-list/

问题描述

206. Reverse Linked List

Easy

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL

Output: 5->4->3->2->1->NULL

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题意

单链表反转。

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思路

维护三个指针head, tmp, ptr，head指向当前节点，tmp指向原链表里当前节点的后继节点，ptr指向原链表里当前节点的前驱节点。顺序遍历原链表，每次将head.next指向ptr，但是原链表的头指针要指向null.

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代码

/**
 * 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)
        {
            return null;
        }
        ListNode ptr = null, tmp = null;
        while (true)
        {
            tmp = head.next;
            if (ptr == null)
            {
                ptr = head;
                ptr.next = null;
            }
            else
            {
                head.next = ptr;
                ptr = head;
            }
            if (tmp == null)
            {
                return head;
            }
            head = tmp;
        }
    }
}