92. Reverse Linked List II（链表局部逆序**）

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.

class Solution {
public:
ListNode* reverseBetween(ListNode* head, int m, int n) {
if(head == NULL || m == n)
return head;

ListNode** pp = &head;
for(int i=1; i<m; ++i)
pp = &((*pp)->next);

ListNode* end = *pp;
ListNode* prev = NULL;
for(int i=m; i<=n; ++i){
ListNode* cur = (*pp);
(*pp) = (*pp)->next;
cur->next = prev;
prev = cur;
}

end->next = *pp;
*pp = prev;

return head;
}

};