Recursion Subset, Subsequences, String

1. skipChar Method: Excluding Specific Characters

Function: Generates subsequences of a given string while excluding occurrences of a specific character.

public static void skipChar(String p, String up) {
    if (up.isEmpty()) {
        System.out.println(p);
        return;
    }
    if (up.charAt(0) == 'a') {
        skipChar(p, up.substring(1));
    } else {
        skipChar(p + up.charAt(0), up.substring(1));
    }
}

2. subseq Method (ArrayList as Argument): Storage for Later Use

Function: Generates all possible subsequences of a given string.

public static void generateSubseq(String p, String up) {
    if (up.isEmpty()) {
        System.out.println(p);
        return;
    }
    // Take (or) include
    generateSubseq(p + up.charAt(0), up.substring(1));
    generateSubseq(p, up.substring(1));
}

3. subseq Method (ArrayList as Argument): Storage for Later Use

Function: Generates all possible subsequences of a given string and stores them in an ArrayList.

public static void subseq(String p, String up, ArrayList<String> result) {
    if (up.isEmpty()) {
        result.add(p);
        return;
    }
    // Include the current character
    subseq(p + up.charAt(0), up.substring(1), result);
    // Exclude the current character
    subseq(p, up.substring(1), result);
}

4. subseq Method (Overloaded): A Variant Approach

Function: An overloaded version of the subseq method also generates subsequences of a given string and stores them in an ArrayList

public static ArrayList<String> subseq(String p, String up) {
    ArrayList<String> result = new ArrayList<>();
    if (up.isEmpty()) {
        result.add(p);
        return result;
    }
    // Capture the results of recursive calls
    ArrayList<String> included = subseq(p + up.charAt(0), up.substring(1));
    ArrayList<String> excluded = subseq(p, up.substring(1));
    // Add the results to the current result
    result.addAll(included);
    result.addAll(excluded);
    return result;
}