Sublist reversal (Linked List — Reversal Sub-pattern)

// Sublist reversal: reverse nodes at positions left..right (1-based), keep the rest.
// Head insertion: walk to the node before the range, then pull the node after
// the pivot to the front of the range, one at a time.
ListNode* reverseBetween(ListNode* head, int left, int right) {
    ListNode dummy(0, head);                   // a fixed node before head, so left = 1 is no special case
    ListNode* before = &dummy;
    for (int i = 1; i < left; i++) before = before->next;   // before = the node just before the range

    ListNode* pivot = before->next;            // the first node of the range; it stays put
    for (int i = left; i < right; i++) {       // right - left flips
        ListNode* moved = pivot->next;         // 1. the node that jumps to the front
        pivot->next = moved->next;             // 2. close the gap it leaves behind
        moved->next = before->next;            // 3. it points at the current front of the range
        before->next = moved;                  // 4. it becomes the new front
    }
    return dummy.next;                         // the dummy's next is the new head
}