截取前 n 个字符：
let str = "Hello, Swift!"
let prefix = str.prefix(5)
print(prefix) // "Hello"
截取后 n 个字符：
let str = "Hello, Swift!"
let suffix = str.suffix(6)
print(suffix) // "Swift!"
截取指定范围内的字符：
let str = "Hello, Swift!"
let range = str.index(str.startIndex, offsetBy: 7)..<str.endIndex
let subString = str[range]
print(subString) // "Swift!"
截取从某个位置到末尾的字符：
let str = "Hello, Swift!"
let index = str.index(str.startIndex, offsetBy: 7)
let subString = str[index...]
print(subString) // "Swift!"
从最后一个指定“.”字符开始截取，到字符串结束：文章来源：https://www.toymoban.com/news/detail-634264.html
let str = "http://www.free.com/346345647457.png"
if let dotIndex = str.lastIndex(of: ".") {
let fileExtension = str[dotIndex+1..<str.endIndex]
print(fileExtension) // "png"文章来源地址https://www.toymoban.com/news/detail-634264.html
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