Adding recursive permutations generating

Author: laucer

Permutations.java

@@ -0,0 +1,60 @@
+package com.jwetherell.algorithms.mathematics;
+
+import java.util.LinkedList;
+
+
+/*
+ * Recursive permutations generating
+ * @author Lucjan Roslanowski <lucjanroslanowski@gmail.com>
+ * 
+ * Example of usage:
+ * int numbers[] = {7,5,3};
+ * LinkedList<LinkedList<Integer>> result = new LinkedList<LinkedList<Integer>>();
+ * getAllPermutations(numbers,result);
+ */
+
+public class Permutations {	
+	
+	public static LinkedList<LinkedList<Integer>> getAllPermutations(final int[] numbers,  LinkedList<LinkedList<Integer>> result){
+		//numbers given in an array are also a permutation
+		LinkedList<Integer> firstPermutation = new LinkedList<>();
+		for(int el : numbers){
+			firstPermutation.add(new Integer(el));
+		}
+		result.add(firstPermutation);
+		//let's permute all elements in array starting from index 0
+		return permute(numbers, 0, result);
+	}
+	
+	public static LinkedList<LinkedList<Integer>> permute(final int[] numbers, 
+			int currentElementIndex,  LinkedList<LinkedList<Integer>> result){
+		if(currentElementIndex == numbers.length - 1){
+			return result;
+		}
+		
+		for(int i = currentElementIndex; i < numbers.length; ++i){
+			//swapping two elements
+			int temp = numbers[i];
+			numbers[i] = numbers[currentElementIndex];
+			numbers[currentElementIndex] = temp;
+			
+			permute(numbers, currentElementIndex + 1,result);
+			
+			//all next permutation found
+			if(i != currentElementIndex){
+				LinkedList<Integer> nextPermutation = new LinkedList<>();
+				for(int j = 0; j < numbers.length; j++)
+					nextPermutation.add(new Integer(numbers[j]));
+				result.add(nextPermutation);
+			}
+			
+			//swapping back two elements 
+			temp = numbers[i];
+			numbers[i] = numbers[currentElementIndex];
+			numbers[currentElementIndex] = temp;
+		}	
+		return result;
+	}
+	
+	
+}