Add postorder binary tree traversal (#3899)

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

@@ -0,0 +1,55 @@
+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;
+
+/**
+ * Given tree is traversed in a 'post-order' way: LEFT -> RIGHT -> ROOT.
+ *
+ * @author Albina Gimaletdinova on 21/02/2023
+ */
+public class PostOrderTraversalTest {
+    @Test
+    public void testNullRoot() {
+        assertEquals(Collections.emptyList(), PostOrderTraversal.recursivePostOrder(null));
+        assertEquals(Collections.emptyList(), PostOrderTraversal.iterativePostOrder(null));
+    }
+
+    /*
+         1
+        / \
+       2   3
+      /\   /\
+     4  5 6  7
+    */
+    @Test
+    public void testPostOrder() {
+        final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{1, 2, 3, 4, 5, 6, 7});
+        List<Integer> expected = List.of(4, 5, 2, 6, 7, 3, 1);
+
+        assertEquals(expected, PostOrderTraversal.recursivePostOrder(root));
+        assertEquals(expected, PostOrderTraversal.iterativePostOrder(root));
+    }
+
+    /*
+         5
+          \
+           6
+            \
+             7
+              \
+               8
+    */
+    @Test
+    public void testPostOrderNonBalanced() {
+        final BinaryTree.Node root = TreeTestUtils.createTree(new Integer[]{5, null, 6, null, 7, null, 8});
+        List<Integer> expected = List.of(8, 7, 6, 5);
+
+        assertEquals(expected, PostOrderTraversal.recursivePostOrder(root));
+        assertEquals(expected, PostOrderTraversal.iterativePostOrder(root));
+    }
+}