Swift String Extention

extension String{
    //下标属性
    subscript(subrange: Range<Int>) -> String {
        get {
            
            var start = advance(startIndex, subrange.startIndex)
            var end = advance(startIndex, subrange.endIndex)
            
            return substringWithRange(Range(start: start, end: end))
            
        }
        
        set {
            
            var start = advance(startIndex, subrange.startIndex)
            var end = advance(startIndex, subrange.endIndex)
            var updateRange = Range(start: start, end: end)
            
            self.replaceRange(updateRange, with: newValue)
            
        }
    }
    
    subscript(index:Int) -> String {
        get{
            return self[Range(start: index, end: index+1)]
        }
        
        set{
            self[Range(start:index,end:index+1)]=newValue;
        }
    }
    
    
    subscript(start:Int,length:Int)->String{
        
        get{
            
            return self[Range(start:start,end:start+length+1)]
            
        }
        
        set{
            
            self[Range(start:start,end:start+length+1)]=newValue;
            
        }
        
    }
    
}