Reverse Linked List II

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

Solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        // (p) -> 1->2->3->4->5->NULL
        //  1     2  3  4  5
        //        p1 p2
        //        1<-2<-3<-4    
        // (p) -> 1->4->3->2->5->NULL
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = dummy;
        ListNode curr = head;
        int p = 1;
        while (p < m) {
            prev = curr;
            curr = curr.next;
            p ++;
        }
        ListNode p1 = prev;
        ListNode p2 = curr;
        while (p <= n) {
            ListNode next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
            p ++;
        }
        p1.next = prev;
        p2.next = curr;
        return dummy.next;
    }
}