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.

Solution:

/** * 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 || m == n) return head;        ListNode *lead = new ListNode(-1);        lead->next = head;        ListNode *top = lead, *pre = lead, *p = head;        int i = 0;        while(p)        {            i++;            if(i < m) top = p;            else if(i > m && i <= n)            {                pre->next = p->next;                p->next = top->next;                top->next = p;                p = pre;            }            else if(i > n) break;            pre = p;            p = p->next;        }        return lead->next;    }};