92.反向链表

题目
反转从位置 m 到 n 的链表。请使用一趟扫描完成反转。

说明:
1 ≤ m ≤ n ≤ 链表长度。

示例:
输入: 1->2->3->4->5->NULL, m = 2, n = 4
输出: 1->4->3->2->5->NULL

思路
参考简书用户安全地带的解法https://www.jianshu.com/p/fbe6012c9e52

#include <cstddef>

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        if (head == NULL) return NULL;
        ListNode* dummy = new ListNode(0);
        dummy->next = head;
        ListNode* previous = dummy;
        for (int i = 0; i < m - 1; i++)
        {
            previous = previous->next;
        }
        ListNode* current = previous->next;
        ListNode* post = current->next;
        for (int i = 0; i < n - m; i++)
        {
            current->next = post->next;
            post->next = previous->next;
            previous->next = post;
            post = current->next;
        }
        return dummy->next;
    }
};