IntersectionPoints.java

package linkedlist;


public class IntersectionPoints {


    /*
     * Given head nodes of two linked lists that may or may not intersect,
     * find out if they intersect and return the point of intersection;
     * return null otherwise.
     * Eg: Input:  29 -> 23 -> 82 -> 11, 12, 17
     *             13 -> 4 -> 12 -> 17
     *     Output: 12
     *
     * */
    public static class LinkedList {

        public static Node head1, head2;

        static class Node {

            int data;
            Node next;

            Node(int d) {
                data = d;
                next = null;
            }
        }

        /*function to get the intersection point of two linked
        lists head1 and head2 */
        int getNode() {
            int c1 = getCount(head1);
            int c2 = getCount(head2);
            int d;

            if (c1 > c2) {
                d = c1 - c2;
                return getIntersectionNode(d, head1, head2);
            } else {
                d = c2 - c1;
                return getIntersectionNode(d, head2, head1);
            }
        }

        /* function to get the intersection point of two linked
         lists head1 and head2 where head1 has d more nodes than
         head2 */
        private int getIntersectionNode(int d, Node node1, Node node2) {
            int i;
            Node current1 = node1;
            Node current2 = node2;
            for (i = 0; i < d; i++) {
                if (current1 == null) {
                    return -1;
                }
                current1 = current1.next;
            }
            while (current1 != null && current2 != null) {
                if (current1.data == current2.data) {
                    return current1.data;
                }
                current1 = current1.next;
                current2 = current2.next;
            }

            return -1;
        }

        /*Takes head pointer of the linked list and
        returns the count of nodes in the list */
        private int getCount(Node node) {
            Node current = node;
            int count = 0;

            while (current != null) {
                count++;
                current = current.next;
            }

            return count;
        }
    }


    public static void main(String[] args) {
        LinkedList list = new LinkedList();


        list.head1 = new LinkedList.Node(3);
        list.head1.next = new LinkedList.Node(6);
        list.head1.next.next = new LinkedList.Node(15);
        list.head1.next.next.next = new LinkedList.Node(15);
        list.head1.next.next.next.next = new LinkedList.Node(30);


        list.head2 = new LinkedList.Node(10);
        list.head2.next = new LinkedList.Node(15);
        list.head2.next.next = new LinkedList.Node(30);

        System.out.println("The node of intersection is " + list.getNode());

    }
}