期货和期权都是杠杆交易工具，但是期权的杠杆要比期货的杠杆高，其原因有以下几点：
The output is always a String, for example H,E,L,L,O,. How could I limit the commas? I want the commas only between letters, for example H,E,L,L,O.
加载中,请稍侯......