Question

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.

Solution

class Solution {
    public static List<String> permutations;
    public List<String> letterCasePermutation(String S) {
        permutations = new ArrayList<String>();
        permute(S, "", 0);
        return permutations;
    }
    
    private static void permute(String s, String sub, int depth){
        if(depth == s.length()){
            System.out.println(sub);
            permutations.add(sub);
            return;
        }
        if(Character.isDigit(s.charAt(depth)))
            permute(s, sub+s.charAt(depth), depth+1);
        else if(s.charAt(depth) == Character.toUpperCase(s.charAt(depth))){
             permute(s, sub+s.charAt(depth), depth+1);
            permute(s, sub+Character.toLowerCase(s.charAt(depth)), depth+1);
        }else{
            permute(s, sub+s.charAt(depth), depth+1);
            permute(s, sub+Character.toUpperCase(s.charAt(depth)), depth+1);
        }
    }
      
}