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C tests accept a wrong next pointer on the last node in Org Chart Row Navigation

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@winter77777 is already working on this.

Since Oct 19, 2026.

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Description

Observed behavior

In Org Chart Row Navigation (Populating Next Right Pointers in Each Node II), the C tests accept a solution whose last next pointer is wrong.

The solution below links each row correctly, then points the last node of the bottom row back to the first node of that row. That node should point to NULL. In C it passes 5/5. The same code in C++ passes 1/5, and every failing case shows "next pointers do not match the tree level order." Java, JavaScript and Python give the same 1/5 with the same message.

static struct Node* rows[128];

struct Node* linkRowNeighbors(struct Node* root) {
    int head = 0, tail = 0;
    if (root) rows[tail++] = root;
    while (head < tail) {
        int end = tail;
        for (int i = head; i < end; i++) {
            rows[i]->next = i + 1 < end ? rows[i + 1] : NULL;
            if (rows[i]->left) rows[tail++] = rows[i]->left;
            if (rows[i]->right) rows[tail++] = rows[i]->right;
        }
        if (tail == end) rows[end - 1]->next = rows[head];
        head = end;
    }
    return root;
}

A wrong next earlier in the tree is not caught cleanly either. If the root's next points to itself, C reports [[1]] for "sparse final level" with no message, so the candidate cannot tell what went wrong.

Expected behavior

C fails a wrong next pointer with "next pointers do not match the tree level order.", like the other four languages. A correct solution still passes 5/5.

Reproduction
  1. Open Org Chart Row Navigation and select C.
  2. Paste the solution above and run tests. All 5 cases pass.
  3. Switch to C++, paste the same logic written with vector<Node*>, and run tests. 4 cases fail with the message above.
CodeTrial version

codetrial 0.1.0 (4faffce)

Environment

OS: Windows 11
Browser: Chromium 155.0.8059.80
Language: C (C++, Java, JavaScript and Python 3 checked for comparison)
Interview loop: Coding
Problem: Org Chart Row Navigation (LeetCode 117)
REACTO or STAR step: Test
Reconnect: None

Sanitized evidence

C, with the solution above:

5/5 test cases passed
OK sparse final level
OK empty tree
OK single node
OK missing middle children
OK one child

C++, with the same logic:

1/5 test cases passed
FAIL sparse final level
input [[1,2,3,4,5,null,7]]
next pointers do not match the tree level order.
OK empty tree
FAIL single node
input [[1]]
next pointers do not match the tree level order.
FAIL missing middle children
input [[1,2,3,null,5,null,7]]
next pointers do not match the tree level order.
FAIL one child
input [[1,2]]
next pointers do not match the tree level order.

Initial analysis at 4faffce:

json_next_tree in the C harness (web/compiler-explorer.js:383 at 4faffce) prints each value first and checks that node's next after. On a mismatch it closes the brackets and returns. The output is then a shorter list that still parses, and if the bad pointer is on the last node, it matches the expected answer exactly.

The C++ and Java harnesses (nextTreeToJson at line 543, nextTreeJson at line 712) and the JavaScript and Python runners (web/runners.js:604, :924) check every pointer and report the message instead of a value.

Fix: check all next pointers before printing, and print the message as a JSON string on a mismatch. json_random_list already reports its error this way in C. I will add a C toolchain test in tests/browser/compiler-explorer.test.js that fails before the fix, and send a pull request.

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Dominant language
Rust
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177
Forks
100
Avg merge
1d 10h
Merged PRs (30d)
135

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