用循环太慢了！
int num = 21 ;
String numStr = String.valueOf(num);
String str ="0000000000";
numStr=str.substring(0, 10-numStr.length())+numStr;
System.out.println(numStr);
string.Format("{0:D10}", 21) -->> 0000000021
string ss = "dwg";
ss = ss.PadLeft(10, 'a');
ss="aaaaaaadwg";