Added recursive&iterative preorder binary tree traversal (#3884)

Added recursive& iterative preorder binary tree traversal

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

@@ -0,0 +1,49 @@
+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 17/02/2023
+ */
+public class PreOrderTraversalTest {
+    @Test
+    public void testNullRoot() {
+        assertEquals(Collections.emptyList(), PreOrderTraversal.recursivePreOrder(null));
+        assertEquals(Collections.emptyList(), PreOrderTraversal.iterativePreOrder(null));
+    }
+
+    /*
+         1
+        / \
+       2   3
+      /\   /\
+     4  5 6  7
+    */
+    @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));
+    }
+
+    /*
+         5
+          \
+           6
+            \
+             7
+              \
+               8
+    */
+    @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));
+    }
+}