feat(content): function signatures + test cases

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

@@ -12,6 +12,24 @@ patterns:
   - backtracking
   - tree-traversal
 
+function_signature: "def binary_tree_paths(root: TreeNode) -> list[str]:"
+
+test_cases:
+  visible:
+    - input: { root: [1, 2, 3, null, 5] }
+      expected: ["1->2->5", "1->3"]
+    - input: { root: [1] }
+      expected: ["1"]
+  hidden:
+    - input: { root: [1, 2, 3] }
+      expected: ["1->2", "1->3"]
+    - input: { root: [1, 2, null, 3, null, 4] }
+      expected: ["1->2->3->4"]
+    - input: { root: [1, 2, 3, 4, 5, 6, 7] }
+      expected: ["1->2->4", "1->2->5", "1->3->6", "1->3->7"]
+    - input: { root: [-1, -2, -3] }
+      expected: ["-1->-2", "-1->-3"]
+
 description: |
   Given the `root` of a binary tree, return *all root-to-leaf paths in **any order***.