java-combinations

java-combinations

Author: hmkcode

java-combinations/Combination.java

@@ -0,0 +1,102 @@
+package com.hmkcode.baloot;
+
+
+
+public class Combination {
+
+	
+	public static void main(String[] args){
+		Object[] elements = new Object[] {'A','B','C','D','E', 'F', 'G', 'H'};
+
+		combination(elements,7);
+	}
+	
+	
+	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
+		// e.g. c(4,2)
+		c(N,K);
+		
+		// get the combination by index 
+		// e.g. 01 --> AB , 23 --> CD
+		int combination[] = new int[K];
+		
+		// position of current index
+		//  if (r = 1)				r*
+		//	index ==>		0	|	1	|	2
+		//	element ==>		A	|	B	|	C
+		int r = 0;		
+		int index = 0;
+		
+		while(r >= 0){
+			// possible indexes for 1st position "r=0" are "0,1,2" --> "A,B,C"
+			// possible indexes for 2nd position "r=1" are "1,2,3" --> "B,C,D"
+			
+			// for r = 0 ==> index < (4+ (0 - 2)) = 2
+			if(index <= (N + (r - K))){
+					combination[r] = index;
+					
+				// if we are at the last position print and increase the index
+				if(r == K-1){
+
+					//do something with the combination e.g. add to list or print
+					print(combination, elements);
+					index++;				
+				}
+				else{
+					// select index for next position
+					index = combination[r]+1;
+					r++;										
+				}
+			}
+			else{
+				r--;
+				if(r > 0)
+					index = combination[r]+1;
+				else
+					index = combination[0]+1;	
+			}			
+		}
+	}
+	
+
+	
+	public 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;
+	}
+	
+	public static int fact(int n)
+	{
+		if(n == 0)
+			return 1;
+		else
+			return n * fact(n-1);
+	}
+	
+
+	public 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);
+	}
+}