Add inorder binary tree traversal (#3898)

src/main/java/com/thealgorithms/datastructures/trees/InorderTraversal.java

@@ -0,0 +1,60 @@
+package com.thealgorithms.datastructures.trees;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Deque;
+import java.util.List;
+
+/**
+ * Given tree is traversed in an 'inorder' way: LEFT -> ROOT -> RIGHT.
+ * Below are given the recursive and iterative implementations.
+ *
+ * Complexities:
+ * Recursive: O(n) - time, O(n) - space, where 'n' is the number of nodes in a tree.
+ *
+ * Iterative: O(n) - time, O(h) - space, where 'n' is the number of nodes in a tree
+ * and 'h' is the height of a binary tree.
+ * In the worst case 'h' can be O(n) if tree is completely unbalanced, for instance:
+ * 5
+ *  \
+ *   6
+ *    \
+ *     7
+ *      \
+ *       8
+ *
+ * @author Albina Gimaletdinova on 21/02/2023
+ */
+public class InorderTraversal {
+    public static List<Integer> recursiveInorder(BinaryTree.Node root) {
+        List<Integer> result = new ArrayList<>();
+        recursiveInorder(root, result);
+        return result;
+    }
+
+    public static List<Integer> iterativeInorder(BinaryTree.Node root) {
+        List<Integer> result = new ArrayList<>();
+        if (root == null) return result;
+
+        Deque<BinaryTree.Node> stack = new ArrayDeque<>();
+        while (!stack.isEmpty() || root != null) {
+            while (root != null) {
+                stack.push(root);
+                root = root.left;
+            }
+            root = stack.pop();
+            result.add(root.data);
+            root = root.right;
+        }
+        return result;
+    }
+
+    private static void recursiveInorder(BinaryTree.Node root, List<Integer> result) {
+        if (root == null) {
+            return;
+        }
+        recursiveInorder(root.left, result);
+        result.add(root.data);
+        recursiveInorder(root.right, result);
+    }
+}

src/test/java/com/thealgorithms/datastructures/trees/InorderTraversalTest.java

@@ -0,0 +1,53 @@
+package com.thealgorithms.datastructures.trees;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * @author Albina Gimaletdinova on 21/02/2023
+ */
+public class InorderTraversalTest {
+    @Test
+    public void testNullRoot() {
+        assertEquals(Collections.emptyList(), InorderTraversal.recursiveInorder(null));
+        assertEquals(Collections.emptyList(), InorderTraversal.iterativeInorder(null));
+    }
+
+    /*
+         1
+        / \
+       2   3
+      /\   /\
+     4  5 6  7
+    */
+    @Test
+    public void testRecursiveInorder() {
+        final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{1, 2, 3, 4, 5, 6, 7});
+        List<Integer> expected = List.of(4, 2, 5, 1, 6, 3, 7);
+
+        assertEquals(expected, InorderTraversal.recursiveInorder(root));
+        assertEquals(expected, InorderTraversal.iterativeInorder(root));
+    }
+
+    /*
+         5
+          \
+           6
+            \
+             7
+              \
+               8
+    */
+    @Test
+    public void testRecursiveInorderNonBalanced() {
+        final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{5, null, 6, null, 7, null, 8});
+        List<Integer> expected = List.of(5, 6, 7, 8);
+
+        assertEquals(expected, InorderTraversal.recursiveInorder(root));
+        assertEquals(expected, InorderTraversal.iterativeInorder(root));
+    }
+}

src/test/java/com/thealgorithms/datastructures/trees/PreOrderTraversalTest.java

@@ -27,8 +27,10 @@ public class PreOrderTraversalTest {
     @Test
     public void testRecursivePreOrder() {
         final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{1, 2, 3, 4, 5, 6, 7});
-        assertEquals(List.of(1, 2, 4, 5, 3, 6, 7), PreOrderTraversal.recursivePreOrder(root));
-        assertEquals(List.of(1, 2, 4, 5, 3, 6, 7), PreOrderTraversal.iterativePreOrder(root));
+        List<Integer> expected = List.of(1, 2, 4, 5, 3, 6, 7);
+
+        assertEquals(expected, PreOrderTraversal.recursivePreOrder(root));
+        assertEquals(expected, PreOrderTraversal.iterativePreOrder(root));
     }
 
     /*
@@ -43,7 +45,9 @@ public class PreOrderTraversalTest {
     @Test
     public void testRecursivePreOrderNonBalanced() {
         final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{5, null, 6, null, 7, null, 8});
-        assertEquals(List.of(5, 6, 7, 8), PreOrderTraversal.recursivePreOrder(root));
-        assertEquals(List.of(5, 6, 7, 8), PreOrderTraversal.iterativePreOrder(root));
+        List<Integer> expected = List.of(5, 6, 7, 8);
+
+        assertEquals(expected, PreOrderTraversal.recursivePreOrder(root));
+        assertEquals(expected, PreOrderTraversal.iterativePreOrder(root));
     }
 }