Leetcode 92. Reverse Linked List II

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        //iterative
        //base case
        if (head == null || head.next == null || m < 1 || m >= n) {
            return head;
        }
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode cur = head;
        //move m-1 to the node before reverse start 
        int count = 1;
        while (count < m) {
            pre = cur;
            cur = cur.next;
            count++;
        }
        ListNode beforeReverse = pre;
        ListNode startReverse = cur;
        
        while (count <= n) {
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
            count++;
        }
        beforeReverse.next = pre;
        startReverse.next = cur;
        return dummy.next;
    }
}