How can I do this in java?

Question

I must combine values in a set in this way:

Input:

{A,B,C,D}

Result:

{AB, ABC, AC, AD, ACD, ABD, ABCD, BC, BD, BCD, CD}

How can I do this?

Answer 1

There are several solutions depending upon the size of the input:
If the input-array is <= 64 and the input doesn't need to be sorted, we can represent each combination as a long, where each bit is 1, if the corresponding element is in the produced output:

void generateCombination(int[] inp){
    long upper_bound = (1 << inp.length);

    for(long rep = 0 ; rep < upper_bound ; rep++){
        for(int i = 0 ; i < inp.length ; i++)
            if(rep & (1 << i) != 0)
                System.out.print(inp[i]);

        System.out.print(",");
    }
}

If the output has to be sorted, we can easily solve this problem using a recursive solution:

void generateCombinations(int[] inp , int[] combination , int at_i , int at_c){
    for(int i = at_i + 1 ; i < inp.length ; ++i)
    {
        combination[at_c] = inp[i];
        System.out.println(Arrays.toString(combination));

        generateCombinations(inp , combination , i , at_c + 1);
    }

    combination[at_c] = 0;
}

//this call will produce all combinations sorted ascending
generateCombinations(inpt , new int[inp.length] , -1 , 0);