[sumnoon]

Problem

LeetCode mode only accepted class Solution with the built-in ListNode/TreeNode. A common learner program such as

struct Node { int val; Node* left; Node* right; Node(int val) { ... } };
void pre_order(Node* root) { ... }

failed immediately with LeetCode mode needs a class Solution. Even past that check, Node* was not a type the harness could build.

What changes

Which function runs

• With a class Solution: its first public method, as before.
• Otherwise: the free function that no other function calls. Recursion into itself is fine, so height() is treated as a helper and isBalanced() runs.
• If several qualify and the code has a main(), the first one that main() calls wins.
• The editor footer says what will run (Calls pre_order(root) for you), and the test-case placeholder lists the parameter names.

Node structs

• Any struct with one scalar value field and one pointer to itself (a list) or two (a binary tree) is built from the test case, whatever the field names (data/next, key/l/r, …). Multi-declarator fields like T *l, *r; work too.
• Nodes are made with the struct's own constructor when it takes the value; otherwise the value field is set directly. Links are always reset to nullptr, because the harness owns the shape.
• Trees use LeetCode's level order with null gaps, via vector<optional<T>>, so string-valued trees work. Printing uses the same generic code.
• ListNode/TreeNode are now just this generator fed LeetCode's definitions, still added when used but not defined.
• A struct that can't be built says why, e.g. builds G* when G has one value field and one G* link (a list) or two (a tree).

Pasting a whole program

• If the code has its own main(), a #define main stackbloom_pasted_main sets it aside without moving any lines, and the test case drives the chosen function.
• If the test case is empty, the program runs unchanged. The hint under the test case says so.

Checks

Compiled and traced eight generated programs through the real tracer:

Case	Output
your pre_order as pasted (constructor does val = val)	four garbage values, faithfully
same with this->val = val, no main	1 2 3 4
same with main, empty test case	own main() runs, 1 2 3 4
struct Node { int data; Node* next; } without a constructor, reverse	[3,2,1]
string tree T *l, *r, main picks mirror over count	["a",null,"b",null,"c"]
height + balanced	true (entry = balanced)
Solution::maxDepth(TreeNode*)	3
Solution::middle(ListNode*)	[3,4,5]

Trace stops only land in the pasted code and the generated main, never in the harness. web/tests/leetcode.test.mjs goes from 5 to 9 tests (17 Node tests in total, all passing). Also checked in the browser: footer, hint, badge and a run of pre_order from root = [1,2,4,3].

README and docs/usage.md are updated.

🤖 Generated with Claude Code