added missing tree.ts to repo

Author: bbonf

frontend/src/app/tree.ts

@@ -0,0 +1,31 @@
+
+interface TreeNode<T> {
+    value: T;
+    children: TreeNode<T>[];
+}
+
+class Tree<T> {
+    _root?: TreeNode<T>;
+
+    get root(): TreeNode<T> {
+        if (!this._root) {
+            throw new Error("Tree is empty")
+        }
+        return this._root;
+    }
+
+    constructor(root?: TreeNode<T>) {
+        this._root = root;
+    }
+
+    static empty<T>(): Tree<T> {
+        return new Tree();
+    }
+
+    static fromJSON<T>(json: any): Tree<T> {
+        // there's no validation of the content
+        return new Tree(json);
+    }
+}
+
+export {Tree, TreeNode};