let sessionId = "this is a test"


 let index = sessionId.index(sessionId.endIndex, offsetBy: -2)
 
 let suffix = sessionId.substring(from: index)

let sessionId = "this is a test"


 let index = sessionId.index(sessionId.startIndex, offsetBy: 2)
 
 let prefix = sessionId.substring(to: index)

 //字符串比较
  //比较字符是否相同
  let s1 = "好的"
  let s2 = "好的"
  if s1 == s2 {
   print("s1 == s2")
  }else{
   print("s1 != s2")
  }
  
  let s3:NSString = "ok"
  let s4:NSString = "ok!"
  if s3.isEqual(to: s4 as String) {
   print("s3 == s4")
  }else{
   print("s3 != s4")
  }
  
  //比较字符串的前缀,后缀
  let array = ["do.docx","good.docx","name.docx","data.json","good.json"]
  for d in array {
   if d.hasPrefix("good"){
    print("前缀为good:\(d)")
   }
  }
  
  for d in array {
   if d.hasSuffix(".json"){
    print("后缀为.json:\(d)")
   }
  }