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package com.hmkcode;

public class Combination {

public static void main(String[] args){

Object[] elements = new Object[] {'A','B','C','D','E'};

combination(elements,3);

}

public static void combination(Object[] elements, int K){

// get the length of the array

// e.g. for {'A','B','C','D'} => N = 4

int N = elements.length;

if(K > N){

System.out.println("Invalid input, K > N");

return;

}

// calculate the possible combinations

c(N,K);

// init combination index array

int combination[] = new int[K];

int r = 0; // index for combination array

int i = 0; // index for elements array

while(r >= 0){

// forward step if i < (N + (r-K))

if(i <= (N + (r - K))){

combination[r] = i;

// if combination array is full print and increment i;

if(r == K-1){

print(combination, elements);

i++;

}

else{

// if combination is not full yet, select next element

i = combination[r]+1;

r++;

}

}

// backward step

else{

r--;

if(r >= 0)

i = combination[r]+1;

}

}

}

private static int c(int n, int r){

int nf=fact(n);

int rf=fact(r);

int nrf=fact(n-r);

int npr=nf/nrf;

int ncr=npr/rf;

System.out.println("C("+n+","+r+") = "+ ncr);

return ncr;

}

private static int fact(int n)

{

if(n == 0)

return 1;

else

return n * fact(n-1);

}

private static void print(int[] combination, Object[] elements){

String output = "";

for(int z = 0 ; z < combination.length;z++){

output += elements[combination[z]];

}

System.out.println(output);

}

}