How to remove duplicates from an unsorted linked list

function removeDups(head) {
  const seen = new Set();
  
  let current = head;
  let prev = null;
  
  while (current) {
    if (seen.has(current.value)) {
      prev.next = current.next;
    } else {
      seen.add(current.value);
      prev = current;
    }
    
    current = current.next;
    
  }
}
// Follow up: How would you solve this problem is a temporary buffer is not allowed?
function deleteDups(head) { // a solution that doesn't use O(n) space
  let current = head;
  while (current) {
    let runner = current;
    while (runner.next) {
      if (runner.next.value === current.value) {
        runner.next = runner.next.next;
      } else {
        runner = runner.next;
      }
    }
    current = current.next;
  }
}
function Node(value) {
  this.next = null;
  this.value = value;
}
Node.prototype.appendToTail = function(data) {
  const end = new Node(data);
  let current = this;
  while (current.next !== null) {
    current = current.next;
  }
  current.next = end;
};