//根据区别难度产生随机字母和数字
for( i=0; readomNumStart.length<4; i) {
(dficult 1) {

//产生随机0-9数字
a = String.valueOf(()(Math.random * 10)) ;
}
(dficult 2) {

//在0-9和abcd中随机产生
String readomWord = {"0","1","2","3","4","5","6","7","8","9","a","b","c","d"};
readomWordIndex = ()(Math.random * 13);
a = readomWord[readomWordIndex];
}
(dficult 3) {

//产生随机字母
char readomLetter = (char)(Math.random *26+'a');
a = String.valueOf(readomLetter) ;
}
(dficult 4) {

//产生随机数字和字母组合
String readomHard = String[20];
readomWordIndex = ()(Math.random * 19);
for( j=0; j<20; j) {
readomWordNum = ()(Math.random * 10);
char readomLetter = (char)(Math.random *26+'a');
(readomWordNum % 2 0) {
readomHard[j] = readomWordNum + "";
}{
readomHard[j] = String.valueOf(readomLetter);
}
}
a = readomHard[readomWordIndex];
}

//这样写代码比较短我也参考了许多人代码大部分都很复杂我不喜欢写那么多代码所以就这样写了还不能知道这样写效率有没有问题在我本机上运行没有什么感觉