相比 [T; N] 形式的数组， Vector 最大的特点就是可以动态调整长度。
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Vec 可以使用 extend 方法进行扩展
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只要为 Vec 实现了 From<T> 特征，那么 T 就可以被转换成 Vec。
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与 String 的切片类似， Vec 也可以使用切片。如果说 Vec 是可变的，那它的切片就是不可变或者说只读的，我们可以通过 & 来获取切片。
在 Rust 中，将切片作为参数进行传递是更常见的使用方式，例如当一个函数只需要可读性时，那传递 Vec 或 String 的切片 &[T] / &str 会更加适合。
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容量 capacity 是已经分配好的内存空间，用于存储未来添加到 Vec 中的元素。而长度 len 则是当前 Vec 中已经存储的元素数量。如果要添加新元素时，长度将要超过已有的容量，那容量会自动进行增长：Rust 会重新分配一块更大的内存空间，然后将之前的 Vec 拷贝过去，因此，这里就会发生新的内存分配( 目前 Rust 的容量调整策略是加倍，例如 2 -> 4 -> 8 ..)。
若这段代码会频繁发生，那频繁的内存分配会大幅影响我们系统的性能，最好的办法就是提前分配好足够的容量，尽量减少内存分配。
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Vec 中的元素必须是相同的类型，例如以下代码会发生错误:
fn main() {
let v = vec![1, 2.0, 3];
}
但是我们可以使用枚举或特征对象来存储不同的类型.
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你可以在这里找到答案(在 solutions 路径下)