92. Reverse Linked List II

Reverse a linked list from position m to n. Do it in-place and in one-pass.
For example:Given1->2->3->4->5->NULL, m = 2 and n = 4,
return1->4->3->2->5->NULL.
Note:Given m, n satisfy the following condition:1 ≤ m ≤ n ≤ length of list.

class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        if(head==NULL||m == n)
          return head;
        
        ListNode* dummy = new ListNode(-1);
        dummy->next = head;
        ListNode* p = head;
        ListNode* q = dummy;
        for(int i=0;i<m-1;i++)
        {
            q = p;
            p = p->next;
        }
        ListNode* pre = NULL;
        ListNode* cur = p;
        
        for(int i=m;i<=n;i++)
        {
            ListNode* pNext = cur->next;
            cur->next = pre;
            pre = cur;
            cur = pNext;
        }
        q->next = pre;
        p->next = cur;
        return dummy->next;
    }
};