feat(content): function signatures + test cases

backend/data/questions/binary-tree-pruning.yaml

@@ -10,6 +10,30 @@ patterns:
   - dfs
   - tree-traversal
 
+function_signature: "def prune_tree(root: TreeNode) -> TreeNode:"
+
+test_cases:
+  visible:
+    - input: { root: [1, null, 0, 0, 1] }
+      expected: [1, null, 0, null, 1]
+    - input: { root: [1, 0, 1, 0, 0, 0, 1] }
+      expected: [1, null, 1, null, 1]
+    - input: { root: [1, 1, 0, 1, 1, 0, 1, 0] }
+      expected: [1, 1, 0, 1, 1, null, 1]
+  hidden:
+    - input: { root: [1] }
+      expected: [1]
+    - input: { root: [0] }
+      expected: null
+    - input: { root: [0, 0, 0] }
+      expected: null
+    - input: { root: [1, 1, 1, 1, 1, 1, 1] }
+      expected: [1, 1, 1, 1, 1, 1, 1]
+    - input: { root: [0, 0, 0, 1] }
+      expected: [0, 0, null, 1]
+    - input: { root: [1, 0, 0, 0, 0, 0, 1] }
+      expected: [1, null, 0, null, 1]
+
 description: |
   Given the `root` of a binary tree, return *the same tree where every subtree (of the given tree) not containing a* `1` *has been removed*.