字符串截取

• 三种截取方法

    let str:String = "天行健，君子以自强不强；地势坤，君子以厚德载物。"
    
    let start = str.startIndex.advancedBy(2)
  
    let stop = str.endIndex.advancedBy(-2)
    
    let strSub1 = str.substringFromIndex(start)
    let strSub11 = str.substringFromIndex('天行健，'.endIndex) //"天行健，"改字符仅仅为占位符，可以用任何相同长度字符串代替
    
    let strSub2 = str.substringToIndex(stop)
    
    let strSub = str.substringWithRange(start..<stop)