swift3.0常用操作包含删除字元串（string），更換字元串，插入字元串

1.删除指定位置的字元串，

let  str = "abcdefg"

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

print(str3)輸出defg

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

print(str4)輸出abcd

2、插入字元串

var  str = "abcdefg"

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

print(str)輸出abc****defg

3、更換字元串

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

print(str2)輸出cdefg