Given 1->2->3->4->5->NULL, m = 2 and n = 4, return 1->4->3->2->5->NULL./**

 * Definition of singly-linked-list:
 * 
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *        this->val = val;
 *        this->next = NULL;
 *     }
 * }
 */


class Solution {
public:
    /*
     * @param head: ListNode head is the head of the linked list 
     * @param m: An integer
     * @param n: An integer
     * @return: The head of the reversed ListNode
     */
     // Dummy -> 1 -> 2 -> 3 -> 4 -> 5 -> NULL
     //  |       |    |    |
     // head    preM mNode postN
     //               |
     //              nNode
     //          
     //          ---------------V
     // Dummy -> 1    2 <- 3 <- 4    5 -> NULL
     //          |    |              |
     //         preM mNode         postN
     //                         |
     //                       nNode
     //               ---------------^
    ListNode * reverseBetween(ListNode * head, int m, int n) {
        if (head == NULL || m > n) {
            return head;
        }
        ListNode *dummy = new ListNode(0);
        dummy->next = head;
        head = dummy;
        for (int i = 1; i < m; i++) {
            if (head == NULL) {
                return NULL;
            }
            head = head->next;
        }

        ListNode *mNode = head->next;
        ListNode *nNode = mNode;
        ListNode *postN = mNode->next;

        for (int i = 0; i < n - m; i++) {
            ListNode *temp = postN->next;
            postN->next = nNode;
            nNode = postN;
            postN = temp;
        }
        mNode->next = postN;
        head->next = nNode;
        return dummy->next;
    }
};