获取结尾两个字符子串：
let sessionId = "this is a test"
let index = sessionId.index(sessionId.endIndex, offsetBy: -2)
let suffix = sessionId.substring(from: index)
最后结果为：“st”
获取开头字符两个：
let sessionId = "this is a test"
let index = sessionId.index(sessionId.startIndex, offsetBy: 2)
let prefix = sessionId.substring(to: index)
最后结果为：“th”
