I've implemented the Heap's algorithm for finding all permutations of the elements of array A:
//A = {1, 2, 3, 4}; B = perms(A) ; num_row(B) = (4!+1) and B[0][0] = 4!;
//This is B.R. Heap's algorithm
public static void perms(int [] A, int [][]B, int n)
{
if (n == 1)
{
int k = B[0][0];
for (int i = 0; i < A.length; i++)
{
B[k + 1][i] = A[i];
}
B[0][0]++;
}
else
{
for (int i = 0; i < n - 1 ;i++)
{
perms(A, B, n-1);
if (n % 2 == 0)
{
swap(A, i, n - 1);
}
else
{
swap(A, 0, n - 1);
}
}
perms(A, B, n - 1);
}
}
public static void swap(int[] A, int i, int j)
{
int temp = A[i];
A[i] = A[j];
A[j] = temp;
}
I'm new to Java. The problem is I want to have B as the output (return) of the function perms(A) , but in this implementation, I have to initialize a int[n! + 1][A.length] B array before calling the function. How can I do it?
Is there anything like private variable or anything in java to help a recursive function to remember a variable from a former call?
Thanks
You can create an "entering" method to recursion like this:
public static int[][] perms(int[] a){
int[][] perms = new int[factorial(a.length)+1][a.length];
perms(a,perms,a.length);
return perms;
}
Method factorial
is well know method and can be found on Google for example
Wondering if n
parameter is neccessary
EDIT
it is not neccessary (above corrected)
EDIT
By my test the k
variable is just incrementing, so I would use static variable like this:
private static int counter = 0;
// your code here, following is a part of your perms method
if (n == 1)
{
for (int i = 0; i < A.length; i++)
{
B[counter][i] = A[i];
}
counter++;
}
//and my code corrected too:
public static int[][] perms(int[] a){
int[][] perms = new int[factorial(a.length)][a.length]; //+1 is not necessary
counter=0; //necessary to call it again
perms(a,perms,a.length);
return perms;
}