Reverse a linked list from position m to n.

class Solution:
    def reverseBetween(self, head: ListNode, m: int, n: int) -> ListNode:
        dummy = ListNode(0)
        dummy.next = head
        prev, curr = dummy, head
        for _ in range(m - 1):
            prev = prev.next
            curr = curr.next

        for _ in range(n - m):
            tmp = curr.next
            curr.next = tmp.next
            tmp.next = prev.next
            prev.next = tmp

        return dummy.next

class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int left, int right) {
        ListNode dummy;
        dummy.next = head;
        ListNode* prev = &dummy;
        for (int i = 0; i < left - 1; i++) {
            prev = prev->next;
            head = head->next;
        }
        for (int i = 0; i < right - left; i ++) {
            ListNode* tmp = head->next;
            head->next = tmp->next;
            tmp->next = prev->next;
            prev->next = tmp;
        }
        return dummy.next;
    }
};