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/**
 * 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.
 * Note:
 * Given m, n satisfy the following condition:
 * 1 ≤ m ≤ n ≤ length of list.
 */
 
 /**
 * 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 || head.next == null || m == n) 
            return head;
        ListNode prev = new ListNode(0);
        prev.next = head;
        head = prev;
        ListNode n1 = head;
        int k = m - 1;
        while(k > 0){
            n1 = n1.next;
            k--;
        }
        // found Mth Node.
        prev = n1;
        n1 = n1.next;
        k = n - m;
        // Start right after the Mth node and reverse it one by one until hit the Nth node.
        while(n1.next != null && k > 0){
            ListNode temp = n1.next;
            n1.next = temp.next;
            temp.next = prev.next;
            prev.next = temp;
            k--;
        }
        return head.next;     
    }
    
    public ListNode reverseBetweenJiuZhang(ListNode head, int m, int n) {
        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;

    }
}