Added Zigzag Traversal of a Binary Tree (#3811)

* Added Zigzag Traversal of a Binary Tree

* fixed file name

Co-authored-by: Albina Gimaletdinova <gimaletdinovaalbina@gmail.com>

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

@@ -0,0 +1,30 @@
+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 11/01/2023
+ */
+public class ZigzagTraversalTest {
+    @Test
+    public void testRootNull() {
+        assertEquals(Collections.emptyList(), ZigzagTraversal.traverse(null));
+    }
+
+    @Test
+    public void testSingleNodeTree() {
+        final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{50});
+        assertEquals(List.of(List.of(50)), ZigzagTraversal.traverse(root));
+    }
+
+    @Test
+    public void testZigzagTraversal() {
+        final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{7, 6, 14, 2, 80, 100});
+        assertEquals(List.of(List.of(7), List.of(14, 6), List.of(2, 80, 100)), ZigzagTraversal.traverse(root));
+    }
+}