92. 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 || !head->next || m == n)
return head;
ListNode *left, *h, *t, *p, *q;
ListNode fake(-1);
fake.next = head;
left = &fake;
int i;
for(i = 1; i < m; i++)
{
left = left->next;
}
h = left->next;
t = h;
p = h->next;
for(i = m+1; i <= n; i++)
{
q = p->next;
p->next = h;
h = p;
p = q;
}
left->next = h;
t->next = p;
return fake.next;
}
};

注意要加fake头结点。