DFS orderings

src/algorithms/graphs/depthFirstSearch.java

@@ -32,10 +32,9 @@
  *          If an adjacency matrix were used, it would cost O(V) to find neighbours for a single vertex, making our
  *          average case time complexity O(V^2) for a connected graph
  *
- * The implementation demonstrates the use of DFS in finding the pre-order (Root, Left, Right) traversal of a binary tree
+ * The implementation demonstrates the use of DFS in finding the order traversals of a binary tree
  * The tree is represented using a custom BinaryTreeNode class
  *
- * TODO: Add new examples, and algo for all orderings
  */
 
 public class depthFirstSearch {
@@ -56,6 +55,36 @@ public static List<Integer> preOrder(BinaryTreeNode root) {
         return traversal;
     }
 
+    public static List<Integer> inOrder(BinaryTreeNode root) {
+        if (root == null) { return new ArrayList<>(); }
+        List<Integer> traversal = new ArrayList<>();
+        traversal.add(root.getVal());
+        if (root.getLeft() == null && root.getRight() == null) {
+            return traversal;
+        } else {
+            // we combine the traversal of the left subtree with the root and the traversal of the right subtree
+            traversal.addAll(inOrder(root.getRight()));
+            List<Integer> left = inOrder(root.getLeft());
+            left.addAll(traversal);
+            return left;
+        }
+    }
+
+    public static List<Integer> postOrder(BinaryTreeNode root) {
+        if (root == null) { return new ArrayList<>(); }
+        List<Integer> traversal = new ArrayList<>();
+        traversal.add(root.getVal());
+        if (root.getLeft() == null && root.getRight() == null) {
+            return traversal;
+        } else {
+            // we combine the traversal of the left and right subtrees with the root
+            List<Integer> leftAndRight = postOrder(root.getLeft());
+            leftAndRight.addAll(postOrder(root.getRight()));
+            leftAndRight.addAll(traversal);
+            return leftAndRight;
+        }
+    }
+
     // call this for visualization of process
     public static List<Integer> preOrderVisualize(BinaryTreeNode root) {
         if (root == null) { return new ArrayList<>(); }

test/algorithms/graphs/depthFirstSearchTest.java

@@ -46,4 +46,76 @@ public void dfs_preOrderTraversal_shouldReturnAccurate() {
         assert secondResult.equals(secondList);
         assert thirdResult.equals(thirdList);
     }
+
+    @Test
+    public void dfs_inOrderTraversal_shouldReturnAccurate() {
+        // empty tree
+        List<Integer> firstList = new ArrayList<>();
+        BinaryTreeNode root1 = null;
+        List<Integer> firstResult = depthFirstSearch.inOrder(root1);
+
+        //standard tree
+        //     1
+        //    / \
+        //   2   3
+        //      / \
+        //     4   5
+        List<Integer> secondList = new ArrayList<>(Arrays.asList(2, 1, 4, 3, 5));
+        BinaryTreeNode rootRight2 = new BinaryTreeNode(3, new BinaryTreeNode(4), new BinaryTreeNode(5));
+        BinaryTreeNode root2 = new BinaryTreeNode(1, new BinaryTreeNode(2), rootRight2);
+        List<Integer> secondResult = depthFirstSearch.inOrder(root2);
+
+        //standard tree 2
+        //          1
+        //         / \
+        //        2   7
+        //       / \
+        //      3   5
+        //     /   /
+        //    4   6
+        List<Integer> thirdList = new ArrayList<>(Arrays.asList(4, 3, 2, 6, 5, 1, 7));
+        BinaryTreeNode rootLeft3 = new BinaryTreeNode(2, new BinaryTreeNode(3, new BinaryTreeNode(4), null), new BinaryTreeNode(5, new BinaryTreeNode(6), null));
+        BinaryTreeNode root3 = new BinaryTreeNode(1, rootLeft3, new BinaryTreeNode(7));
+        List<Integer> thirdResult = depthFirstSearch.inOrder(root3);
+
+        assert firstResult.equals(firstList);
+        assert secondResult.equals(secondList);
+        assert thirdResult.equals(thirdList);
+    }
+
+    @Test
+    public void dfs_postOrderTraversal_shouldReturnAccurate() {
+        // empty tree
+        List<Integer> firstList = new ArrayList<>();
+        BinaryTreeNode root1 = null;
+        List<Integer> firstResult = depthFirstSearch.inOrder(root1);
+
+        //standard tree
+        //     1
+        //    / \
+        //   2   3
+        //      / \
+        //     4   5
+        List<Integer> secondList = new ArrayList<>(Arrays.asList(2, 4, 5, 3, 1));
+        BinaryTreeNode rootRight2 = new BinaryTreeNode(3, new BinaryTreeNode(4), new BinaryTreeNode(5));
+        BinaryTreeNode root2 = new BinaryTreeNode(1, new BinaryTreeNode(2), rootRight2);
+        List<Integer> secondResult = depthFirstSearch.postOrder(root2);
+
+        //standard tree 2
+        //          1
+        //         / \
+        //        2   7
+        //       / \
+        //      3   5
+        //     /   /
+        //    4   6
+        List<Integer> thirdList = new ArrayList<>(Arrays.asList(4, 3, 6, 5, 2, 7, 1));
+        BinaryTreeNode rootLeft3 = new BinaryTreeNode(2, new BinaryTreeNode(3, new BinaryTreeNode(4), null), new BinaryTreeNode(5, new BinaryTreeNode(6), null));
+        BinaryTreeNode root3 = new BinaryTreeNode(1, rootLeft3, new BinaryTreeNode(7));
+        List<Integer> thirdResult = depthFirstSearch.postOrder(root3);
+
+        assert firstResult.equals(firstList);
+        assert secondResult.equals(secondList);
+        assert thirdResult.equals(thirdList);
+    }
 }