/// Removes and returns the first element of the collection.
///
/// - Returns: The first element of the collection if the collection is
/// not empty; otherwise, `nil`.
///
/// - Complexity: O(1)
@inlinable public mutating func popFirst() -> Element?
Swift_集合 let element = arr.`popLast() `/无`popFirst()方法`移除最后一个元素,并返回该元素【移除并返回集合的最后一个元素。】注意无popFirst()方法。