LeetCode 92: Reverse Linked List II

/**
 * 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;
};