Reverse Linked List II

/** * 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(m == n) {            return head;        }        ListNode dummy = new ListNode(0);        dummy.next = head;        ListNode prev = dummy, curr = head;        ListNode revPrev = dummy;        for(int i = 1; i <= n; i++) {            if(i == m) {                revPrev = prev;            }            if(i <= n && i > m) {                prev.next = curr.next;                curr.next = revPrev.next;                revPrev.next = curr;                curr = prev;            }            prev = curr;            curr = curr.next;        }        return dummy.next;    }}

Time: O(n)

Space: O(1)