prob15.js

const arrayToList = (arr) => {
  let head = {};
  let list = head;
  for(let i = 0; i < arr.length; i++){
    list.val = arr[i];
    list.rest = {};
    list = list.rest;
  }
  
  return list;
}

const listToArray = (list) => {
  let workingList = list;
  let arr = [];
  while(list.value != null){
  	arr.push(list.value);
    workingList = workingList.rest;
  }
  return arr;
}

const prepend = (list, val) => {
  return {
    value: val,
    rest: list
  };
}

const nth = (list, index) => {
  let workingList = list;
  if(index < 0)
    return undefined;
  
  let cur = 0;
  while(cur < index){
    if(workingList == null)
      return undefined;
    workingList = workingList.rest;
    cur++;
  }
  
  if(workingList == null || workingList.value == null)
      return undefined;
  return list.value;
}

const recurNth = (list, index, curIndex) => {
  
  if(list == null)
    return undefined;
  
  if(index == curIndex){
    if(list.value == null)
      return undefined;
    return list.value;
  }
  
  return recurNth(list.rest, index, curIndex + 1);
}