/**
 * Definition for singly-linked list.
 */
 function ListNode(val, next) {
    this.val = (val===undefined ? 0 : val)
    this.next = (next===undefined ? null : next)
 }

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var deleteDuplicates = function(head) {
  let prev = null;
  let current = head;
  let found = {};
  while (current != null) {
     if (current.val in found) {
         prev.next = current.next;
         current = current.next;
     } else {
         found[current.val] = true;
         prev = current;
         current = current.next;
     }
  }
  return head;
};

const printLinkedList = (ll) => {
    while (ll != null) {
       console.log(ll.val);
       ll = ll.next;
    }
};

console.log("Expected: 1->2");
let ll = new ListNode(1, new ListNode(1, new ListNode(2)));
console.log(`Got:`);
printLinkedList(deleteDuplicates(ll));