string permutation with upcase and lowcase

Give a string, which only contains a-z. List all the permutation of upcase and lowcase. 
For example, str = "ab",  the output should be 
"ab", "aB", "Ab", "AB" 
for str = "abc", the output should be 
"abc", "abC", "aBc", "aBC", "Abc", "AbC", "ABc", "ABC" 

[解题思路]

本题与其他permutation题目区别在于结果中每位的字符都是固定的，仅仅是大小写的区别

因而不需要循环来遍历，使字符串的每一位可以遍历任意一个值

public class Solution {
    public static void main(String[] args) {
        List<String> result = new ArrayList<String>();
        String tmp = "";
        permutation(tmp, 0, 4, result);
        System.out.println(result);
    }
    private static void permutation(String tmp, int depth, int len,
            List<String> result) {
        if (depth == len) {
            result.add(tmp);
            return;
        }

        tmp += (char) ('a' + depth);
        permutation(tmp, depth + 1, len, result);
        tmp = tmp.substring(0, tmp.length() - 1);

        tmp += (char) ('A' + depth);
        permutation(tmp, depth + 1, len, result);
        tmp = tmp.substring(0, tmp.length() - 1);

    }
}