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BAEL-1773 Find the middle element of a Linked List (#4463)

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* BAEL-1773 - find middle element of linked list

* changes from review

* changes from review

* find middle element in linked list

* typo

* changes from CR

* BAEL-1773 formatting
This commit is contained in:
Marcos Lopez Gonzalez 2018-06-12 03:12:07 +02:00 committed by Predrag Maric
parent b62c35bd7a
commit 2a07b03b69
3 changed files with 76 additions and 83 deletions
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58
core-java/src/main/java/com/baeldung/linkedlist/LinkedList.java
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@ -1,58 +0,0 @@
package com.baeldung.linkedlist;
/**
* Implementation of a singly linked list.
*/
public class LinkedList {
private Node head;
private Node tail;
public Node head() {
return head;
}
public void add(String data) {
Node newNode = new Node(data);
if (head == null) {
head = newNode;
tail = newNode;
} else {
tail.next = newNode;
tail = newNode;
}
}
public static class Node {
private Node next;
private String data;
public Node(String data) {
this.data = data;
}
public String data() {
return data;
}
public void setData(String data) {
this.data = data;
}
public boolean hasNext() {
return next != null;
}
public Node next() {
return next;
}
public void setNext(Node next) {
this.next = next;
}
public String toString() {
return this.data;
}
}
}

25
core-java/src/main/java/com/baeldung/linkedlist/MiddleElementLookup.java
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@ -1,22 +1,23 @@
package com.baeldung.linkedlist;
import java.util.LinkedList;
import java.util.Optional;
import com.baeldung.linkedlist.Node;
public class MiddleElementLookup {
public static String findMiddleElementLinkedList(LinkedList<String> linkedList) {
public static Optional<String> findMiddleElementLinkedList(LinkedList<String> linkedList) {
if (linkedList == null || linkedList.isEmpty()) {
return null;
return Optional.empty();
}
return linkedList.get((linkedList.size() - 1) / 2);
return Optional.ofNullable(linkedList.get((linkedList.size() - 1) / 2));
}
public static String findMiddleElementFromHead(Node head) {
public static Optional<String> findMiddleElementFromHead(Node head) {
if (head == null) {
return null;
return Optional.empty();
}
// calculate the size of the list
@ -33,17 +34,17 @@ public class MiddleElementLookup {
current = current.next();
}
return current.data();
return Optional.ofNullable(current.data());
}
public static String findMiddleElementFromHead1PassRecursively(Node head) {
public static Optional<String> findMiddleElementFromHead1PassRecursively(Node head) {
if (head == null) {
return null;
return Optional.empty();
}
MiddleAuxRecursion middleAux = new MiddleAuxRecursion();
findMiddleRecursively(head, middleAux);
return middleAux.middle.data();
return Optional.ofNullable(middleAux.middle.data());
}
private static void findMiddleRecursively(Node node, MiddleAuxRecursion middleAux) {
@ -63,9 +64,9 @@ public class MiddleElementLookup {
middleAux.length--;
}
public static String findMiddleElementFromHead1PassIteratively(Node head) {
public static Optional<String> findMiddleElementFromHead1PassIteratively(Node head) {
if (head == null) {
return null;
return Optional.empty();
}
Node slowPointer = head;
@ -78,7 +79,7 @@ public class MiddleElementLookup {
slowPointer = slowPointer.next();
}
return slowPointer.data();
return Optional.ofNullable(slowPointer.data());
}
private static class MiddleAuxRecursion {

76
core-java/src/test/java/com/baeldung/linkedlist/MiddleElementLookupUnitTest.java
View File

@ -1,6 +1,7 @@
package com.baeldung.linkedlist;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import java.util.LinkedList;
@ -10,34 +11,83 @@ public class MiddleElementLookupUnitTest {
@Test
public void whenFindingMiddleLinkedList_thenMiddleFound() {
assertEquals("3", MiddleElementLookup.findMiddleElementLinkedList(createLinkedList(5)));
assertEquals("2", MiddleElementLookup.findMiddleElementLinkedList(createLinkedList(4)));
assertEquals("3", MiddleElementLookup
.findMiddleElementLinkedList(createLinkedList(5))
.get());
assertEquals("2", MiddleElementLookup
.findMiddleElementLinkedList(createLinkedList(4))
.get());
}
@Test
public void whenFindingMiddleFromHead_thenMiddleFound() {
assertEquals("3", MiddleElementLookup.findMiddleElementFromHead(createNodesList(5)));
assertEquals("2", MiddleElementLookup.findMiddleElementFromHead(createNodesList(4)));
assertEquals("3", MiddleElementLookup
.findMiddleElementFromHead(createNodesList(5))
.get());
assertEquals("2", MiddleElementLookup
.findMiddleElementFromHead(createNodesList(4))
.get());
}
@Test
public void whenFindingMiddleFromHead1PassRecursively_thenMiddleFound() {
assertEquals("3", MiddleElementLookup.findMiddleElementFromHead1PassRecursively(createNodesList(5)));
assertEquals("2", MiddleElementLookup.findMiddleElementFromHead1PassRecursively(createNodesList(4)));
assertEquals("3", MiddleElementLookup
.findMiddleElementFromHead1PassRecursively(createNodesList(5))
.get());
assertEquals("2", MiddleElementLookup
.findMiddleElementFromHead1PassRecursively(createNodesList(4))
.get());
}
@Test
public void whenFindingMiddleFromHead1PassIteratively_thenMiddleFound() {
assertEquals("3", MiddleElementLookup.findMiddleElementFromHead1PassIteratively(createNodesList(5)));
assertEquals("2", MiddleElementLookup.findMiddleElementFromHead1PassIteratively(createNodesList(4)));
assertEquals("3", MiddleElementLookup
.findMiddleElementFromHead1PassIteratively(createNodesList(5))
.get());
assertEquals("2", MiddleElementLookup
.findMiddleElementFromHead1PassIteratively(createNodesList(4))
.get());
}
@Test
public void whenListEmptyOrNull_thenMiddleNull() {
assertEquals(null, MiddleElementLookup.findMiddleElementLinkedList(null));
assertEquals(null, MiddleElementLookup.findMiddleElementFromHead(null));
assertEquals(null, MiddleElementLookup.findMiddleElementFromHead1PassIteratively(null));
assertEquals(null, MiddleElementLookup.findMiddleElementFromHead1PassRecursively(null));
public void whenListEmptyOrNull_thenMiddleNotFound() {
// null list
assertFalse(MiddleElementLookup
.findMiddleElementLinkedList(null)
.isPresent());
assertFalse(MiddleElementLookup
.findMiddleElementFromHead(null)
.isPresent());
assertFalse(MiddleElementLookup
.findMiddleElementFromHead1PassIteratively(null)
.isPresent());
assertFalse(MiddleElementLookup
.findMiddleElementFromHead1PassRecursively(null)
.isPresent());
// empty LinkedList
assertFalse(MiddleElementLookup
.findMiddleElementLinkedList(new LinkedList<>())
.isPresent());
// LinkedList with nulls
LinkedList<String> nullsList = new LinkedList<>();
nullsList.add(null);
nullsList.add(null);
assertFalse(MiddleElementLookup
.findMiddleElementLinkedList(nullsList)
.isPresent());
// nodes with null values
assertFalse(MiddleElementLookup
.findMiddleElementFromHead(new Node(null))
.isPresent());
assertFalse(MiddleElementLookup
.findMiddleElementFromHead1PassIteratively(new Node(null))
.isPresent());
assertFalse(MiddleElementLookup
.findMiddleElementFromHead1PassRecursively(new Node(null))
.isPresent());
}
private static LinkedList<String> createLinkedList(int n) {