1986

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

class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        if(head == null || m >= n) return head;

        ListNode dummy = new ListNode(0);
        dummy.next = head;
        head = dummy;

        for(int i = 1; i < m; i++) {
            head = head.next;
        }

        ListNode premNode = head;
        ListNode mNode = head.next;
        ListNode nNode = head.next;
        ListNode postnNode = nNode.next;

        for(int i = m; i < n; i++) {
            ListNode temp = postnNode.next;
            postnNode.next = nNode;
            nNode = postnNode;
            postnNode = temp;
        }

        premNode.next = nNode;
        mNode.next = postnNode;

        return dummy.next;
    }
}