fibLinear = n => {
  let fibNMinus2 = 0;
  let fibNMinus1 = 1;
  let fib = n;
  for (let i = 1; i < n; i++) {
    fib = fibNMinus1 + fibNMinus2;
    fibNMinus2 = fibNMinus1;
    fibNMinus1 = fib;
  }
  return fib;
}
O(n) in time, O(1) in space and doesn't use recursion

@param Number
@return Number