BAEL-1773 Find the middle element of a Linked List. (#4350)

* BAEL-1773 - find middle element of linked list

* changes from review

* changes from review

core-java/src/main/java/com/baeldung/linkedlist/LinkedList.java

@@ -0,0 +1,58 @@
+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;
+        }
+    }
+}

core-java/src/main/java/com/baeldung/linkedlist/MiddleElementLookup.java

@@ -0,0 +1,79 @@
+package com.baeldung.linkedlist;
+
+import com.baeldung.linkedlist.LinkedList.Node;
+
+public class MiddleElementLookup {
+
+    public static String findMiddleElement(Node head) {
+        if (head == null) {
+            return null;
+        }
+
+        // calculate the size of the list
+        Node current = head;
+        int size = 1;
+        while (current.hasNext()) {
+            current = current.next();
+            size++;
+        }
+
+        // iterate till the middle element
+        current = head;
+        for (int i = 0; i < (size - 1) / 2; i++) {
+            current = current.next();
+        }
+
+        return current.data();
+    }
+
+    public static String findMiddleElement1PassRecursively(Node head) {
+        if (head == null) {
+            return null;
+        }
+
+        MiddleAuxRecursion middleAux = new MiddleAuxRecursion();
+        findMiddleRecursively(head, middleAux);
+        return middleAux.middle.data();
+    }
+
+    private static void findMiddleRecursively(Node node, MiddleAuxRecursion middleAux) {
+        if (node == null) {
+            // reached the end
+            middleAux.length = middleAux.length / 2;
+            return;
+        }
+        middleAux.length++;
+        findMiddleRecursively(node.next(), middleAux);
+
+        if (middleAux.length == 0) {
+            // found the middle
+            middleAux.middle = node;
+        }
+
+        middleAux.length--;
+    }
+
+    public static String findMiddleElement1PassIteratively(Node head) {
+        if (head == null) {
+            return null;
+        }
+
+        Node slowPointer = head;
+        Node fastPointer = head;
+
+        while (fastPointer.hasNext() && fastPointer.next()
+            .hasNext()) {
+            fastPointer = fastPointer.next()
+                .next();
+            slowPointer = slowPointer.next();
+        }
+
+        return slowPointer.data();
+    }
+
+    private static class MiddleAuxRecursion {
+        Node middle;
+        int length = 0;
+    }
+
+}

core-java/src/test/java/com/baeldung/linkedlist/MiddleElementLookupUnitTest.java

@@ -0,0 +1,58 @@
+package com.baeldung.linkedlist;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+public class MiddleElementLookupUnitTest {
+
+    @Test
+    public void whenFindingMiddle_thenMiddleFound() {
+        String middle = MiddleElementLookup.findMiddleElement(createList(5).head());
+        assertEquals("3", middle);
+
+        middle = MiddleElementLookup.findMiddleElement(createList(4).head());
+        assertEquals("2", middle);
+    }
+
+    @Test
+    public void whenFindingMiddle1PassRecursively_thenMiddleFound() {
+        String middle = MiddleElementLookup.findMiddleElement1PassRecursively(createList(5).head());
+        assertEquals("3", middle);
+
+        middle = MiddleElementLookup.findMiddleElement1PassRecursively(createList(4).head());
+        assertEquals("2", middle);
+    }
+
+    @Test
+    public void whenFindingMiddle1PassIteratively_thenMiddleFound() {
+        String middle = MiddleElementLookup.findMiddleElement1PassIteratively(createList(5).head());
+        assertEquals("3", middle);
+
+        middle = MiddleElementLookup.findMiddleElement1PassIteratively(createList(4).head());
+        assertEquals("2", middle);
+    }
+
+    @Test
+    public void whenListEmptyOrNull_thenMiddleNull() {
+        String middle = MiddleElementLookup.findMiddleElement(null);
+        assertEquals(null, middle);
+
+        middle = MiddleElementLookup.findMiddleElement1PassIteratively(null);
+        assertEquals(null, middle);
+
+        middle = MiddleElementLookup.findMiddleElement1PassRecursively(null);
+        assertEquals(null, middle);
+    }
+
+    private static LinkedList createList(int n) {
+        LinkedList list = new LinkedList();
+
+        for (int i = 1; i <= n; i++) {
+            list.add(String.valueOf(i));
+        }
+
+        return list;
+    }
+
+}