LintCode 翻转链表 II题目代码

/**
 * Definition for ListNode
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param ListNode head is the head of the linked list 
     * @oaram m and n
     * @return: The head of the reversed ListNode
     */
    public ListNode reverseBetween(ListNode head, int m , int n) {
        // write your code
        if (m >= n || head == null) {
            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 premNode = head;
        ListNode mNode = head.next;
        ListNode nNode = mNode, postnNode = mNode.next;
        for (int i = m; i < n; i++) {
            if (postnNode == null) {
                return null;
            }
            ListNode temp = postnNode.next;
            postnNode.next = nNode;
            nNode = postnNode;
            postnNode = temp;
        }
        mNode.next = postnNode;
        premNode.next = nNode;
        
        return dummy.next;
    }
}