Reverse Linked List II.java

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.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
    	if (head == null) return head;
	int start = 1, end = 0;
	ListNode s = head;
	ListNode p = null;
	while (start++ < m && s != null) {
	    p = s;
	    s = s.next;
	}
	ListNode cur = null;
	ListNode next = null;
	ListNode prev = null;
	end = m;
	cur = s;
	while (end++ <= n && cur != null) {
	    next = cur.next;
	    cur.next = prev;
	    prev = cur;
	    cur = next;
	}
	s.next = cur;
	if (p != null) {
	    p.next = prev;
	    return head;
	} else {
	    return prev;
	}
    }
}