LeetCode之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) {
ListNode *sentry = new ListNode(0);
sentry->next = head;
ListNode * p, *q;
p = sentry; q = sentry;
int i = 1;
while (i<m){
p = p->next;
i++;
}
i = 1;
q = p->next;
while (i<n-m+1){
ListNode * tmp = q->next;
q->next = tmp->next;
tmp->next = p->next;
p->next = tmp;
i++;
}
return sentry->next;
}
};