Question

I need to write a function which finds the possible fixed length combinations of a string. The need is that not all combis are required. For example, if the string is “abcde”, and we need combis of length 3, then the function must return the following:

abc
abd
abe
acd
ace
ade
bcd
bde
bce
cde

and none else. I have been trying for it using recursion but things have not worked out as expected. I have also seen some similar questions but could not get much out of them. Algorithm or code(C, C++, Java), any help is welcome. Thanks.!

Note: The combinations need to be ordered. That is, the characters should follow the same order as in the input string.

Answer 1

I expect it’s possible to do better, but this is what I could come up with quickly.

I used a List to preserve order, and had to avoid duplicates awkwardly. Using a Set instead would allow us to skip the result.contains(s) check, but a better algorithm might avoid the duplicates in a cleaner way.

private static List<String> substrings(int i, String input) {
    List<String> result = new ArrayList<String>();
    if (i == 0)
        return result;

    String first = input.substring(0, i);
    result.add(first);

    if (input.length() == i) {
        return result;
    }

    // Recursively find substrings of next smaller length not including the first character
    List<String> tails = substrings(i-1, input.substring(1));

    // Append first char to each result of the recursive call.
    for (String sub: tails) {
        String s = input.substring(0, 1) + sub;
        if (!(result.contains(s)))
            result.add(s);
    }

    // Add all substring of current length not including first character
    result.addAll(substrings(i, input.substring(1)));
    return result;
}