将字符的出现次数计入字符串

鉴于 String 和 Character

let text = "Hello World"
let char: Character = "o"

我们可以计算 Character 出现在 String 中的次数

let sensitiveCount = text.characters.filter { $0 == char }.count // case-sensitive
let insensitiveCount = text.lowercaseString.characters.filter { $0 == Character(String(char).lowercaseString) } // case-insensitive