1.删除指定位置的字符串，

let  str = "abcdefg"

let str3 = str.substring(from: str.index(str.endIndex, offsetBy: -4))

print(str3)输出defg

let str4 = str.substring(to: str.index(str.startIndex, offsetBy: 4))

print(str4)输出abcd


2、插入字符串

var  str = "abcdefg"

str.insert(contentsOf: "****".characters, at: str.index(str.startIndex, offsetBy: 3))

print(str)输出abc****defg

3、更换字符串

var  str = "abcdefg"

let str2 = str.replacingOccurrences(of: "ab", with: "")

print(str2)输出cdefg