/**
 * A recursion demo using a helper method.
 */
public class HasSubsetSum {

    /**
     * The primary method to check whether some subset of a
     * sum to target. Just a wrapper around the helper method
     * that actually does the recursive work.
     */
    public static boolean hasSubsetSum(int[] a, int target) {
        return subsetHelper(a, target, 0, 0);
    }
    
    /**
     * The recursive helper method for hasSubsetSum. Uses extra parameters
     * to track all needed information across recursive calls.
     */
    private static boolean subsetHelper(int[] a, int target, int nextIndex, int sumSoFar) {
        if (sumSoFar == target) {
            return true;
        } else if (sumSoFar > target) {
            return false;
        } else if (nextIndex == a.length) {
            return false;
        } else {
            boolean with = subsetHelper(a, target, nextIndex + 1, sumSoFar + a[nextIndex]);
            boolean without = subsetHelper(a, target, nextIndex + 1, sumSoFar);
            return with || without;
        }
    }
    
    // test the method
    public static void main(String[] args) {
        int[] vals = {3, 2, 7, 3, 10};
        System.out.println("13 possible? " + hasSubsetSum(vals, 13));
        System.out.println("14 possible? " + hasSubsetSum(vals, 14));
    }   

}