How to Generate All Unique Combinations from Multiple Lists of Varying Lengths in Java?

How to Generate All Combinations from Multiple Lists

Question:

Given a variable number of lists with varying lengths, determine how to generate a single list containing all unique combinations of elements from the input lists.

For instance:

Consider the following lists:

X: [A, B, C]
Y: [W, X, Y, Z]

Desired output:

12 unique combinations:

[AW, AX, AY, AZ, BW, BX, BY, BZ, CW, CX, CY, CZ]

Java Implementation:

Using Recursion:

<code class="java">import java.util.List;

public class Permutations {

    public static void main(String[] args) {
        List<List<Character>> lists = List.of(List.of('A', 'B', 'C'), List.of('W', 'X', 'Y', 'Z'));
        List<String> result = new ArrayList<>();
        generatePermutations(lists, result, 0, "");
        System.out.println(result);
    }

    private static void generatePermutations(List<List<Character>> lists, List<String> result, int depth, String current) {
        if (depth == lists.size()) {
            result.add(current);
            return;
        }

        for (int i = 0; i < lists.get(depth).size(); i++) {
            generatePermutations(lists, result, depth + 1, current + lists.get(depth).get(i));
        }
    }
}</code>

How it Works:

This function uses recursion to explore all possible combinations. It maintains a current string representing the partial combination so far and increments the depth to indicate which list is being examined. The base case occurs when all lists have been exhausted, at which point the current partial combination is added to the result list. Otherwise, the function iterates through the current list and recursively calls itself, adding each character to the current combination and advancing the depth.

Example Usage:

<code class="java">Input: lists = [[A, B, C], [W, X, Y, Z]]

Output: [AW, AX, AY, AZ, BW, BX, BY, BZ, CW, CX, CY, CZ]</code>

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