Solution.java

import java.util.*;

class Solution
{
    public List<String> letterCasePermutation(String S)
    {
        final List<String> result = new ArrayList<>();
        _findPermutations(S.toCharArray(), 0, result, Arrays.asList(""));
        return result;
    }

    private void _findPermutations(char[] chars, int i, List<String> result, List<String> partialResult)
    {
        if (i == chars.length)
        {
            result.addAll(partialResult);
            return;
        }
        final char chr = chars[i];
        final Set<Character> permutations = new HashSet<>();
        if (Character.isAlphabetic(chr))
        {
            permutations.add(Character.toUpperCase(chr));
            permutations.add(Character.toLowerCase(chr));
        }
        else
        {
            permutations.add(chr);
        }
        final ArrayList<String> newPartialResult = new ArrayList<>();
        permutations.forEach(c -> partialResult.forEach(s -> newPartialResult.add(s + c)));
        _findPermutations(chars, i + 1, result, newPartialResult);
    }
}