Feat | Interview | #13 Build Comments Tree

README.md

@@ -131,4 +131,5 @@ Interview Questions
 - [Task #9](src/interview/render-items/no-async) - _Render Items: No Async_
 - [Task #10](src/interview/render-items/with-async) - _Render Items: With Async_
 - [Task #11](src/interview/build-tree/no-replies-sort) - _Build Tree: No Replies Sort_
-- [Task #12](src/interview/build-tree/with-replies-sort) - _Build Tree: With Replies Sort_
+- [Task #12](src/interview/build-tree/with-replies-sort) - _Build Tree: With Replies Sort_
+- [Task #13](src/interview/build-comments-tree) - _Build Comments Tree_

src/interview/build-comments-tree/solution.test.ts

@@ -0,0 +1,111 @@
+import { buildCommentsTree } from './solution';
+
+describe('Build Comments Tree | Interview | Testcases', () => {
+  test('#1 Builds the comments tree from the example', () => {
+    const comments = [
+      { id: 1, text: 'A', level: 0 },
+      { id: 2, text: 'B', level: 1 },
+      { id: 3, text: 'C', level: 2 },
+      { id: 4, text: 'D', level: 1 },
+      { id: 5, text: 'E', level: 0 },
+    ];
+
+    expect(buildCommentsTree(comments)).toEqual([
+      {
+        id: 1,
+        text: 'A',
+        level: 0,
+        children: [
+          {
+            id: 2,
+            text: 'B',
+            level: 1,
+            children: [{ id: 3, text: 'C', level: 2 }],
+          },
+          { id: 4, text: 'D', level: 1 },
+        ],
+      },
+      { id: 5, text: 'E', level: 0 },
+    ]);
+  });
+
+  test('#2 Uses the last previous comment from the parent level', () => {
+    const comments = [
+      { id: 1, text: 'First root', level: 0 },
+      { id: 2, text: 'First child', level: 1 },
+      { id: 3, text: 'Second root', level: 0 },
+      { id: 4, text: 'Second child', level: 1 },
+      { id: 5, text: 'Second grandchild', level: 2 },
+    ];
+
+    expect(buildCommentsTree(comments)).toEqual([
+      {
+        id: 1,
+        text: 'First root',
+        level: 0,
+        children: [{ id: 2, text: 'First child', level: 1 }],
+      },
+      {
+        id: 3,
+        text: 'Second root',
+        level: 0,
+        children: [
+          {
+            id: 4,
+            text: 'Second child',
+            level: 1,
+            children: [{ id: 5, text: 'Second grandchild', level: 2 }],
+          },
+        ],
+      },
+    ]);
+  });
+
+  test('#3 Supports arbitrary nesting depth', () => {
+    const comments = [
+      { id: 1, text: 'Level 0', level: 0 },
+      { id: 2, text: 'Level 1', level: 1 },
+      { id: 3, text: 'Level 2', level: 2 },
+      { id: 4, text: 'Level 3', level: 3 },
+    ];
+
+    expect(buildCommentsTree(comments)).toEqual([
+      {
+        id: 1,
+        text: 'Level 0',
+        level: 0,
+        children: [
+          {
+            id: 2,
+            text: 'Level 1',
+            level: 1,
+            children: [
+              {
+                id: 3,
+                text: 'Level 2',
+                level: 2,
+                children: [{ id: 4, text: 'Level 3', level: 3 }],
+              },
+            ],
+          },
+        ],
+      },
+    ]);
+  });
+
+  test('#4 Does not mutate the source array or its comments', () => {
+    const comments = [
+      { id: 1, text: 'Root', level: 0 },
+      { id: 2, text: 'Child', level: 1 },
+    ];
+    const originalComments = comments.map((comment) => ({ ...comment }));
+
+    buildCommentsTree(comments);
+
+    expect(comments).toEqual(originalComments);
+  });
+
+  test('#5 Returns an empty array for an empty input', () => {
+    expect(buildCommentsTree([])).toEqual([]);
+  });
+});

src/interview/build-comments-tree/solution.ts

@@ -0,0 +1,58 @@
+interface Comment {
+  id: number;
+  text: string;
+  level: number;
+}
+
+interface Tree extends Comment {
+  children?: Tree[];
+}
+
+/**
+ * @param comments - array of elements with id, text & level
+ *
+ * @returns array of comments with `level = 0`.
+ * Element with `level N` must be inserted into `children` property of last previous element with `N - 1 level`.
+ * `comments` array is not mutated
+ *
+ * @example
+ *
+ * const comments = [
+ *   { id: 1, text: 'A', level: 0 },
+ *   { id: 2, text: 'B', level: 1 },
+ *   { id: 3, text: 'C', level: 2 },
+ *   { id: 4, text: 'D', level: 1 },
+ *   { id: 5, text: 'E', level: 0 },
+ * ];
+ *
+ * console.log(JSON.stringify(buildCommentsTree(comments), null, 2));
+ *
+ * // Expected structure:
+ * // A
+ * // ├─ B
+ * // │  └─ C
+ * // └─ D
+ * // E
+ */
+export const buildCommentsTree = (comments: Comment[]): Tree[] => {
+  const roots: Tree[] = [];
+  const commentsMap = new Map<number, Tree>();
+  for (const comment of comments) {
+    const node: Tree = { ...comment };
+    commentsMap.set(comment.level, node);
+    if (comment.level === 0) {
+      roots.push(node);
+      continue;
+    }
+
+    const parent = commentsMap.get(comment.level - 1);
+    if (!parent) {
+      continue;
+    }
+
+    parent.children ??= [];
+    parent.children.push(node);
+  }
+
+  return roots;
+};