feat(content): function signatures + test cases

backend/data/questions/binary-tree-postorder-traversal.yaml

@@ -11,6 +11,26 @@ patterns:
   - dfs
   - tree-traversal
 
+function_signature: "def postorder_traversal(root: TreeNode) -> list[int]:"
+
+test_cases:
+  visible:
+    - input: { root: [1, null, 2, 3] }
+      expected: [3, 2, 1]
+    - input: { root: [] }
+      expected: []
+    - input: { root: [1] }
+      expected: [1]
+  hidden:
+    - input: { root: [1, 2, 3, 4, 5, null, 8, null, null, 6, 7, 9] }
+      expected: [4, 6, 7, 5, 2, 9, 8, 3, 1]
+    - input: { root: [1, 2, null, 3, null, 4] }
+      expected: [4, 3, 2, 1]
+    - input: { root: [1, null, 2, null, 3] }
+      expected: [3, 2, 1]
+    - input: { root: [5, 3, 7, 2, 4, 6, 8] }
+      expected: [2, 4, 3, 6, 8, 7, 5]
+
 description: |
   Given the `root` of a binary tree, return *the postorder traversal of its nodes' values*.