Merge sort on linked list

Sorting/src/main/LinkedListMergeSort.java

@@ -0,0 +1,67 @@
+package main;
+
+public class LinkedListMergeSort {
+
+  public ListNode mergeSortList(ListNode head) {
+    // Base condition of the recursion. If the list is empty or if only one element is present,
+    // return the element
+    if (head == null || head.next == null) {
+      return head;
+    }
+
+    // Use slow and fast pointer method. Slow points to the first element and fast points to the
+    // second element. Then onwards, fast is incremented twice for each increment in slow pointer.
+    // This way, we can split the linked list in 2 halves.
+    ListNode slow = head;
+    ListNode fast = head.next;
+    while (fast != null) {
+      fast = fast.next;
+      if (fast != null) {
+        slow = slow.next;
+        fast = fast.next;
+      }
+    }
+
+    // Create two lists, slow becomes the head of first half and fast becomes the head of the second
+    // half. Both halves are passed to the recursion.
+    fast = slow.next;
+    slow.next = null;
+    slow = head;
+    ListNode h1 = mergeSortList(slow);
+    ListNode h2 = mergeSortList(fast);
+
+    // Merge the result of sorted lists
+    ListNode result = merge(h1, h2);
+    return result;
+  }
+
+  private ListNode merge(ListNode h1, ListNode h2) {
+    ListNode p1 = h1;
+    ListNode p2 = h2;
+    // Fake head is the previous node of head of the merged linked list. This trick is used when you
+    // don't want to allocate separate memory for storing the merging result.
+    ListNode fakeHead = new ListNode(0);
+    ListNode p = fakeHead;
+    while (p1 != null && p2 != null) {
+      if (p1.value <= p2.value) {
+        p.next = p1;
+        p1 = p1.next;
+      } else {
+        p.next = p2;
+        p2 = p2.next;
+      }
+      p = p.next;
+    }
+    if (p1 != null) {
+      // Attach remaining elements to the result
+      p.next = p1;
+    }
+    if (p2 != null) {
+      // Attach remaining elements to the result
+      p.next = p2;
+    }
+    // Return fake head's next element which is the actual head of the merged linked list.
+    return fakeHead.next;
+  }
+
+}

Sorting/src/main/ListNode.java

@@ -0,0 +1,11 @@
+package main;
+
+public class ListNode {
+  public final int value;
+  public ListNode next;
+
+  public ListNode(int value) {
+    this.value = value;
+    this.next = null;
+  }
+}

Sorting/src/test/LinkedListTest.java

@@ -0,0 +1,36 @@
+package test;
+
+import static org.junit.Assert.fail;
+import main.LinkedListMergeSort;
+import main.ListNode;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class LinkedListTest {
+  private ListNode n1;
+
+  @Before
+  public void setup() throws Exception {
+    n1 = new ListNode(15);
+    ListNode n2 = new ListNode(2);
+    ListNode n3 = new ListNode(11);
+
+    n1.next = n2;
+    n2.next = n3;
+    n3.next = null;
+  }
+
+  @Test
+  public void testLinkedListMergeSort() throws Exception {
+    LinkedListMergeSort llm = new LinkedListMergeSort();
+    ListNode result = llm.mergeSortList(n1);
+    ListNode temp = result;
+    while (temp.next != null) {
+      if (temp.value > temp.next.value) {
+        fail("LinkedList is not sorted...");
+      }
+      temp = temp.next;
+    }
+  }
+}