Merge pull request #91 from jaebradley/reverse-linked-list

linked list reverser

Author: jaebradley

src/main/java/problems/impl/LinkedListReverser.java

@@ -0,0 +1,29 @@
+package problems.impl;
+
+public class LinkedListReverser {
+    public static class ListNode {
+        int val;
+        ListNode next;
+        ListNode(int x) { val = x; }
+    }
+
+    public static ListNode reverse(ListNode head) {
+        if (head == null) {
+            return null;
+        }
+
+        ListNode current = head;
+        ListNode previous = null;
+        ListNode next;
+
+        while (current.next != null) {
+            next = current.next;
+            current.next = previous;
+            previous = current;
+            current = next;
+        }
+
+        current.next = previous;
+        return current;
+    }
+}

src/test/java/problems/impl/LinkedListReverserTest.java

@@ -0,0 +1,40 @@
+package problems.impl;
+
+import org.junit.Test;
+import problems.impl.LinkedListReverser.ListNode;
+
+import static org.junit.Assert.*;
+
+public class LinkedListReverserTest {
+
+    @Test
+    public void nullForEmptyLinkedList() {
+        assertNull(LinkedListReverser.reverse(null));
+    }
+
+    @Test
+    public void headForSingleValueList() {
+        ListNode head = new ListNode(0);
+
+        assertEquals(head, LinkedListReverser.reverse(head));
+    }
+
+    @Test
+    public void reverseValues() {
+        ListNode head = new ListNode(0);
+        ListNode one = new ListNode(1);
+        ListNode two = new ListNode(2);
+        ListNode three = new ListNode(3);
+        head.next = one;
+        one.next = two;
+        two.next = three;
+
+        ListNode reversed = LinkedListReverser.reverse(head);
+
+        assertEquals(3, reversed.val);
+        assertEquals(2, reversed.next.val);
+        assertEquals(1, reversed.next.next.val);
+        assertEquals(0, reversed.next.next.next.val);
+        assertNull(reversed.next.next.next.next);
+    }
+}