【使用】：返回两个整数之间的距离。
值得注意的是：就算数组abc一个元素都没有，abc.distance(from: 1, to: 10)，也不会报错，仍然可用，值等于9。
/// Returns the distance between two indices.
/// - Parameters:
/// - start: A valid index of the collection.
/// - end: Another valid index of the collection. If `end` is equal to
/// `start`, the result is zero.
/// - Returns: The distance between `start` and `end`.
@inlinable public func distance(from start: Int, to end: Int) -> Int
let abc = ["1", "2", "3"]
let distance = abc.distance(from: 1, to: 10)
print(distance)
等于：9
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