【LeetCode】Reverse Linked List II

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
struct Node
{
    ListNode* head;
    ListNode* tail;
    Node(ListNode* begin, ListNode* end): head(begin), tail(end) {}
};

class Solution {
public:
    ListNode *reverseBetween(ListNode *head, int m, int n) {
        //find begin
        ListNode* tail = head;
        ListNode* cur = head;
        ListNode* begin = NULL;
        ListNode* end = NULL;
        ListNode* pre = NULL;
        int tmp = m;
        
        while(--tmp)//--tmp
        {
            pre = cur;
            cur = cur->next;
        }
        begin = cur;
        tmp = n-m;
        while(tmp--)//tmp--
            cur = cur->next;
        end = cur;
        
        while(tail->next != NULL)
            tail = tail->next;
        
        //4 cases
        if(begin == head)
        {//result->head is head
            if(end == tail)
            {
                //reverse all, return result->head
                Node* result = reverse(begin, end);
                result->tail->next = NULL;
                return result->head;
            }
            else
            {
                //reverse from head to end, end->next=post, and return result->head
                ListNode* post = end->next;
                end->next = NULL;
                Node* result = reverse(begin, end);
                result->tail->next = post;
                return result->head;
            }
        }
        else
        {//head is head
           if(end == tail)
            {
                //reverse from begin to tail, pre->next = result->head, and return head
                Node* result = reverse(begin, end);
                pre->next = result->head;
                result->tail->next = NULL;
                return head;
            }
            else
            {
                //reverse from begin to end, pre->next = begin, end->next = post, and return head
                ListNode* post = end->next;
                end->next = NULL;
                Node* result = reverse(begin, end);
                pre->next = result->head;
                result->tail->next = post;
                return head;
            } 
        }
    }
    
    Node* reverse(ListNode* begin, ListNode* end)
    {
        Node* result = new Node(begin, end);
        if(begin == end)
            return result;  //no change
        else if(begin->next == end)
        {
            end->next = begin;
            begin->next = NULL;
            result->head = end;
            result->tail = begin;
            return result;
        }
        else
        {
            ListNode* pre = begin;
            ListNode* cur = pre->next;
            ListNode* post = cur->next;
            while(post != NULL)
            {
                cur->next = pre;
                pre = cur;
                cur = post;
                post = post->next;
            }
            cur->next = pre;
            begin->next = NULL;
            result->head = cur;
            result->tail = begin;
            return result;
        }
    }
};