Remove nth node from end of list

remove-nth-list-node.ts

/**
 * 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 removeNthFromEnd(head: ListNode | null, n: number): ListNode | null {
    // Traverse to the nth item from the end of the list by using a forward-looking pointer.
    let result = head;
    let slow = head;
    let fast = head;
    let parent = null;

    // Set fast pointer. When this pointer becomes null, we are on the node to delete.
    for (let i=0; i<n; i++) {
        fast = fast.next;
    }

    while (slow != null) {
        if (!fast) {
            // slow is the node to be deleted.
            if (parent || slow.next) {
                // Remove the current node.
                if (parent) {
                    parent.next = slow.next;
                }
                else {
                    // Remove the first node. We are on the first node, but there are still children.
                    result = slow.next;
                }
                break;
            }
            else {
                result = null;
                break;
            }
        }

        parent = slow;
        slow = slow.next;
        fast = fast.next;
    }

    return result;
};