[LeetCode] Reverse Linked List II

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.

/** * Definition for singly-linked list. * 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 == nullptr || m == n)   return head;        ListNode *p = head, *q = nullptr;        for(int i = 0; i < m-1; i++)        {            q = p;            p = p->next;        }        ListNode *end = p, *pre = p;        p = p->next;        for(int i = m+1; i <= n; i++)        {            ListNode *pNext = p->next;            p->next = pre;            pre = p;            p = pNext;        }        end->next = p;        if(q)            q->next = pre;        else            head = pre;        return head;    }};