마지막 두 원소

https://school.programmers.co.kr/learn/courses/30/lessons/181927

function solution(num_list) {
  const length = num_list.length;
  const lastNum = num_list[length-1];
  const notLast = num_list[length-2];
    const addValue = lastNum > notLast ? lastNum - notLast : lastNum * 2;
    return [...num_list, addValue]
}