LeetCode – BackTracking – Letter Case Permutation

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 between 1 and 12.
• S will consist only of letters or digits.

class Solution {
    public List<String> letterCasePermutation(String S) {
        List<String> results = new ArrayList<String>();
        getCombinations(0,results,S,"");
        return results;
    }
    
    public void getCombinations(int str_len, List<String> results, String S, String current) {        
        if (str_len == S.length()){
            results.add(current);
            return;
        }
        char c = S.charAt(str_len);
        if(Character.isLetter(c)){
            getCombinations(str_len+1,results,S,current+Character.toLowerCase(c));
            getCombinations(str_len+1,results,S,current+Character.toUpperCase(c));
        }else{
            getCombinations(str_len+1,results,S,current+c);
        }
    }
}