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;
}
};