784-字母大小写全排列

public List<String> letterCasePermutation(String S) {
    backtrace(S, 0, new StringBuilder());
    return res;
}

private List<String> res = new ArrayList<>();
public void backtrace(String s, int index, StringBuilder path){
    //结束条件：path.length == s.length
    if (path.length() == s.length()){
        res.add(path.toString());
        return ;
    }
    //开始枚举回溯
    if (Character.isDigit(s.charAt(index))){    //数字
        path.append(s.charAt(index));           //选择
        backtrace(s, index + 1, path);    		//递归
        path.deleteCharAt(path.length() - 1);   //撤销
    }else {     //字母
        //大写
        path.append(String.valueOf(s.charAt(index)).toUpperCase()); //选择
        backtrace(s, index + 1, path);    		//递归
        path.deleteCharAt(path.length() - 1);   //撤销
        //小写
        path.append(String.valueOf(s.charAt(index)).toLowerCase()); //选择
        backtrace(s, index + 1, path);    		//递归
        path.deleteCharAt(path.length() - 1);   //撤销
    }
}