reverseBetween.h

/**
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) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        ListNode dummy(0);
        dummy.next = head;
        ListNode *preM, *pre = &dummy;
        for(int i = 1; i <= n; ++i){
            if(i == m) preM = pre;
            if(i > m && i <= n){
                pre->next = head->next;
                head->next = preM->next;
                preM->next = head;
                head = pre;
            }
            pre = head;
            head = head->next;
        }
        return dummy.next;
    }
};