/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */
 
function reverseBetween(head: ListNode | null, left: number, right: number): ListNode | null {
    
    if (head === null)
        return null;
    
    let currPtr = head, prevPtr = null;
    
    while (left > 1) {
        prevPtr = currPtr;
        currPtr = currPtr.next;
        
        left--;
        right--;
    }
    
    let tailPtr = currPtr, connPtr = prevPtr, thirdPtr: ListNode | null = null;
    
    while (right > 0) {
        thirdPtr = currPtr.next;
        currPtr.next = prevPtr;
        prevPtr = currPtr;
        currPtr = thirdPtr;
        
        right--;
    }
    
    if (connPtr)
        connPtr.next = prevPtr;
    else
        head = prevPtr;
    
    tailPtr.next = currPtr;
    
    return head;
};