Add SymmetricTree task solution

Author: andrei-punko

src/main/java/by/andd3dfx/tree/SymmetricTree.java

@@ -0,0 +1,51 @@
+package by.andd3dfx.tree;
+
+import lombok.AllArgsConstructor;
+import lombok.NoArgsConstructor;
+
+/**
+ * <pre>
+ * Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).
+ *
+ * Example 1:
+ * <img src="https://assets.leetcode.com/uploads/2021/02/19/symtree1.jpg"/>
+ * Input: root = [1,2,2,3,4,4,3]
+ * Output: true
+ *
+ * Example 2:
+ * <img src="https://assets.leetcode.com/uploads/2021/02/19/symtree2.jpg"/>
+ * Input: root = [1,2,2,null,3,null,3]
+ * Output: false
+ * </pre>
+ */
+public class SymmetricTree {
+
+    public static boolean isSymmetric(TreeNode root) {
+        var left = root.left;
+        var right = root.right;
+        return equals(left, right);
+    }
+
+    private static boolean equals(TreeNode node1, TreeNode node2) {
+        if (node1 == null) {
+            return node2 == null;
+        }
+        if (node2 == null) {
+            return false;
+        }
+        return node1.val == node2.val && equals(node1.left, node2.right) && equals(node1.right, node2.left);
+    }
+
+    @AllArgsConstructor
+    @NoArgsConstructor
+    public static class TreeNode {
+
+        int val;
+        TreeNode left;
+        TreeNode right;
+
+        TreeNode(int val) {
+            this.val = val;
+        }
+    }
+}

src/test/java/by/andd3dfx/iterators/RecursiveIteratorTest.java

@@ -1,14 +1,12 @@
 package by.andd3dfx.iterators;
 
-import org.junit.Ignore;
-import org.junit.Test;
+import static org.assertj.core.api.Assertions.assertThat;
 
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 import java.util.NoSuchElementException;
-
-import static org.assertj.core.api.Assertions.assertThat;
+import org.junit.Test;
 
 public class RecursiveIteratorTest {
 

src/test/java/by/andd3dfx/tree/SymmetricTreeTest.java

@@ -0,0 +1,27 @@
+package by.andd3dfx.tree;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import by.andd3dfx.tree.SymmetricTree.TreeNode;
+import org.junit.Test;
+
+public class SymmetricTreeTest {
+
+    @Test
+    public void isSymmetricMirroredLeftNRight() {
+        TreeNode root = new TreeNode(1,
+            new TreeNode(2, new TreeNode(3), new TreeNode(4)),
+            new TreeNode(2, new TreeNode(4), new TreeNode(3))
+        );
+        assertThat(SymmetricTree.isSymmetric(root)).isTrue();
+    }
+
+    @Test
+    public void isSymmetricTheSameLeftNRight() {
+        TreeNode root2 = new TreeNode(1,
+            new TreeNode(2, null, new TreeNode(3)),
+            new TreeNode(2, null, new TreeNode(3))
+        );
+        assertThat(SymmetricTree.isSymmetric(root2)).isFalse();
+    }
+}