LeetCode92 Reverse Linked List II

题目：

       Reverse a linked list from position m to n. Do it in one-pass.

       Note: 1 ≤ m ≤ n ≤ length of list.

       Example:

       Input: 1->2->3->4->5->NULL, m = 2, n = 4

       Output: 1->4->3->2->5->NULL

public class ReverseLinkedListII {

    public static ListNode reverseBetween(ListNode head, int m, int n) {
        if(head == null) return null;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = dummy;
        for(int i = 0; i<m-1; i++) pre = pre.next;

        ListNode start = pre.next;

        for(int i=0; i<n-m; i++)
        {
            ListNode t=start.next;
            start.next=t.next;
            t.next=pre.next;
            pre.next=t;

        }
        return dummy.next;
    }

    public static ListNode createLinkedList(int arr[], int n){
        if (n == 0)
            return null;
        ListNode head = new ListNode(arr[0]);
        ListNode cur = head;
        for (int i = 1; i < n; i++) {
            cur.next = new ListNode(arr[i]);
            cur = cur.next;
        }

        return head;
    }

    public static void printLinkedList(ListNode head){
        ListNode cur = head;
        while (cur != null){
            System.out.print(cur.val + " -> ");
            cur = cur.next;
        }
        System.out.println("NULL");
    }

    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        int m = 2, n = 5;
        ListNode head = createLinkedList(arr, arr.length);
        ListNode reverseHead = reverseBetween(head, m, n);
        printLinkedList(reverseHead);
    }
}

class ListNode {
    public int val;
    public ListNode next;
    public ListNode(int x) {
        val = x;
    }
}