將字元的出現次數計入字串

鑑於 String 和 Character

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

我們可以計算 Character 出現在 String 中的次數

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