feat: Binary Tree to String algorithm added (Leetcode 606) (#6727)

* feat: BinaryTreeToString.java Added with tests

* fix: javadoc added to BinaryTreeToString

* fix: link to leetcode added

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

@@ -0,0 +1,57 @@
+package com.thealgorithms.datastructures.trees;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for the BinaryTreeToString class.
+ */
+public class BinaryTreeToStringTest {
+
+    @Test
+    public void testTreeToStringBasic() {
+        BinaryTree tree = new BinaryTree();
+        tree.put(1);
+        tree.put(2);
+        tree.put(3);
+        tree.put(4);
+
+        BinaryTreeToString converter = new BinaryTreeToString();
+        String result = converter.tree2str(tree.getRoot());
+
+        // Output will depend on insertion logic of BinaryTree.put()
+        // which is BST-style, so result = "1()(2()(3()(4)))"
+        Assertions.assertEquals("1()(2()(3()(4)))", result);
+    }
+
+    @Test
+    public void testSingleNodeTree() {
+        BinaryTree tree = new BinaryTree();
+        tree.put(10);
+
+        BinaryTreeToString converter = new BinaryTreeToString();
+        String result = converter.tree2str(tree.getRoot());
+
+        Assertions.assertEquals("10", result);
+    }
+
+    @Test
+    public void testComplexTreeStructure() {
+        BinaryTree.Node root = new BinaryTree.Node(10);
+        root.left = new BinaryTree.Node(5);
+        root.right = new BinaryTree.Node(20);
+        root.right.left = new BinaryTree.Node(15);
+        root.right.right = new BinaryTree.Node(25);
+
+        BinaryTreeToString converter = new BinaryTreeToString();
+        String result = converter.tree2str(root);
+
+        Assertions.assertEquals("10(5)(20(15)(25))", result);
+    }
+
+    @Test
+    public void testNullTree() {
+        BinaryTreeToString converter = new BinaryTreeToString();
+        Assertions.assertEquals("", converter.tree2str(null));
+    }
+}