題目：

Given a string S, we can transform every letter individually to be lowercase or uppercase to create another string.  Return a list of all possible strings we could create.

Examples:
Input: S = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]

Input: S = "3z4"
Output: ["3z4", "3Z4"]

Input: S = "12345"
Output:
 
 ["12345"]

Note:

• S will be a string with length at most 12.
• S will consist only of letters or digits.

程式碼：

class Solution {
    public List<String> letterCasePermutation(String S) {
        res = new ArrayList<String>();
        func(S.toCharArray(), 0, "");
        return res;
    }
    List<String> res;
    public void func(char[] str, int index, String ans){
        if(index == str.length){
            res.add(ans);
            return;
        }
        if(Character.isLetter(str[index])){
            func(str, index+1, ans+str[index]);
            char c;
            if(str[index] >= 'a' && str[index] <= 'z') c = (char)(str[index] + 'A' - 'a');
            else c = (char)(str[index] + 'a' - 'A');
            func(str, index+1, ans+c);
        }
        else func(str, index+1, ans+str[index]);
    }
}