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Find Intersection

/*************************************************************************** * Author: Isai Damier * Title: Singly Linked List * Project: geekviewpoint * Package: datastructure * * Description: A LinkedList is a data structure that allows access * to a collection of data using pointers/references. While an * array can also be defined as above, LinkedLists and arrays differ * in how they are stored in memory and in the operations they * allow. Unlike an array that must be stored in a block of memory, * the nodes of a LinkedList can be stored anywhere because each * node has a reference to the node that succeeds it. Because the * nodes are stored so loosely, inserting nodes into a LinkedList * is easy; whereas in an array, all the succeeding elements must * be shifted. Of course, insertion also means changing the size of * the array, which means creating the entire array anew. * * Perhaps the greatest beauty of LinkedList is that it allows * accessing an entire sequence of nodes using only one variable: * a reference to the first node in the sequence. * * Countless operations can be performed on LinkedLists. Following * are a few, ranging from the common to the very interesting. **************************************************************************/ public class SinglyLinkedList { Node head = null; Node tail = null; /***************************************************************** * Time Complexity of Solution: * O(n). * * Description: Find the node (intersection) where the two given * LinkedList coalesce. * * Technical Details: The idea is that two linked lists form a Y-shaped * structure. The structure may be degenerated. As shown below, the use * of a hashmap greatly reduces the complexity of the problem. * * * To keep things simple, we consider t2 and t2 equal iff they both * point to the same memory location. A broader definition of * equality: Two nodes t2 and t1 are equal if they and their respective * sublists are equal. *****************************************************************/ static public Node findIntersection(Node head1, Node head2) { Map<Node, Boolean> intersect = new HashMap<Node, Boolean>(); for (Node t = head1; null != t; t = t.next) { intersect.put(t, true); } //first duplicate is intersection for (Node t = head2; null != t; t = t.next) { if (null != intersect.get(t)) { return t; } } return null; } }

public class SinglyLinkedListTest { /** * Test of findIntersection method, of class SinglyLinkedList. */ @Test public void testFindIntersection() { System.out.println("findIntersection"); int[] in_1 = {29, 14, 35, 2, 1, 12, 6, 7, 4, 8, 3, 0, 16, 19, 11}; int[] in_2 = {99, 78, 8, 3, 23}; SinglyLinkedList list_1 = new SinglyLinkedList(); SinglyLinkedList list_2 = new SinglyLinkedList(); for (int i = 0; i < in_1.length; i++) { list_1.addToTail(in_1[i]); } assertEquals(in_1.length, list_1.size()); for (int i = 0; i < in_2.length; i++) { list_2.addToTail(in_2[i]); } assertEquals(in_2.length, list_2.size()); //set intersection: list_1 and list_2 form a Y at element 6 int exp = 6; Node found = list_1.find(exp); assertNotNull(found); list_2.addAllToTail(found); assertEquals(14, list_2.size()); Node n = SinglyLinkedList.findIntersection(list_1.head, list_2.head); assertEquals(exp, n.data); } }