insertBetween method
在数组之间插入元素
Implementation
List<E> insertBetween(E value) {
if (isEmpty) return this;
List<E> newList = [];
for (int i = 0; i < length; i++) {
newList.add(this[i]);
if (i < length - 1) newList.add(value);
}
return newList;
}