diff --git a/tests/browser/compiler-explorer.test.js b/tests/browser/compiler-explorer.test.js
index fa2d9dcf..93b42d8b 100644
--- a/tests/browser/compiler-explorer.test.js
+++ b/tests/browser/compiler-explorer.test.js
@@ -812,6 +812,215 @@ public:
     });
   }
 
+  it(
+    "C next-pointer tree harness checks every next pointer, up to 6000 nodes",
+    { skip: noCc },
+    async () => {
+      const dir = mkdtempSync(join(tmpdir(), "codetrial-c-next-"));
+      const judge = judges["populating-next-right-pointers-in-each-node-ii"];
+      const completeTree = (label, size, value = (n) => n) => {
+        const levels = [];
+        for (let first = 1; first <= size; first *= 2) {
+          const count = Math.min(first * 2, size + 1) - first;
+          levels.push(
+            Array.from({ length: count }, (_, i) => value(first + i)),
+          );
+        }
+        const input = [Array.from({ length: size }, (_, i) => value(i + 1))];
+        return { label, input, expected: levels };
+      };
+      // 64 nodes fill the harness's first queue exactly, so they never need
+      // it to grow, and all of them hold the same value, so only a node's
+      // address tells a repeat from a sibling. A candidate case can be as
+      // large as the problem's 6000-node limit, past what a fixed 4096-entry
+      // queue holds.
+      const exactFit = completeTree("exact fit", 64, () => 1);
+      const spec = {
+        ...judge,
+        cases: [...judge.cases, exactFit, completeTree("full-size tree", 6000)],
+      };
+      // The bottom row's last node can point back to that row's first node
+      // instead of NULL, which printing the values alone hides, or take
+      // itself as a left child, which passes every next check while the
+      // tree never ends.
+      const linkRows = (lastNext, lastLeft) => `static struct Node* rows[6000];
+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 = ${lastNext};
+            rows[end - 1]->left = ${lastLeft};
+        }
+        head = end;
+    }
+    return root;
+}`;
+      const actuals = async (name, code, flags = []) => {
+        const source = join(dir, `${name}.c`);
+        const binary = join(dir, name);
+        writeFileSync(source, generateHarness("c", spec, code));
+        await run("cc", ["-std=c17", ...flags, source, "-o", binary]);
+        return parseCompilerResults(
+          (await run(binary, [], { encoding: "utf8", timeout: 10000 })).stdout,
+        ).results.map((result) =>
+          result.error === undefined ? result.actual : { error: result.error },
+        );
+      };
+      try {
+        assert.deepEqual(
+          await actuals("correct", linkRows("NULL", "NULL")),
+          spec.cases.map((testCase) => testCase.expected),
+        );
+        const mismatch = {
+          error: "next pointers do not match the tree level order.",
+        };
+        assert.deepEqual(
+          await actuals("ring", linkRows("rows[head]", "NULL")),
+          [mismatch, [], mismatch, mismatch, mismatch, mismatch, mismatch],
+        );
+        // Only the root's left child is wrong: its next points back at the
+        // root instead of across to its sibling. That is an upper level, a
+        // pointer that is not NULL, and in the exact fit a node with the
+        // sibling's value, so checking only the last pointer of each level,
+        // only whether a pointer is NULL, or only values would all miss it.
+        const linkAllRows = linkRows("NULL", "NULL").replace(
+          "linkRowNeighbors",
+          "linkAllRows",
+        );
+        const interior = `${linkAllRows}
+struct Node* linkRowNeighbors(struct Node* root) {
+    linkAllRows(root);
+    if (root && root->left && root->right) root->left->next = root;
+    return root;
+}`;
+        assert.deepEqual(
+          await actuals("interior", interior),
+          spec.cases.map((testCase) =>
+            testCase.expected[1]?.length === 2 ? mismatch : testCase.expected,
+          ),
+        );
+        const twice = { error: "Returned tree must not reach a node twice." };
+        // The root's right child is its left child again, a repeat inside the
+        // level being queued rather than one already walked.
+        const sharedChild = `${linkAllRows}
+struct Node* linkRowNeighbors(struct Node* root) {
+    linkAllRows(root);
+    if (root && root->left) root->right = root->left;
+    return root;
+}`;
+        assert.deepEqual(
+          await actuals("shared-child", sharedChild),
+          spec.cases.map((testCase) =>
+            testCase.expected.length > 1 ? twice : testCase.expected,
+          ),
+        );
+        // Every realloc in the harness is the queue's, so renaming it to one
+        // that always fails takes the full-size case down the grow failure,
+        // while the exact fit never asks to grow.
+        const failingRealloc = `void* failing_realloc(void* ptr, size_t size) {
+    (void)ptr;
+    (void)size;
+    return NULL;
+}
+`;
+        const noMemory = {
+          error: "Out of memory while checking next pointers.",
+        };
+        assert.deepEqual(
+          await actuals(
+            "no-memory",
+            failingRealloc + linkRows("NULL", "NULL"),
+            ["-Drealloc=failing_realloc"],
+          ),
+          [
+            ...judge.cases.map((testCase) => testCase.expected),
+            exactFit.expected,
+            noMemory,
+          ],
+        );
+        // A repeated child is refused before the queue would grow for it, so
+        // the exact fit reports the repeat rather than the failed grow.
+        assert.deepEqual(
+          await actuals(
+            "no-memory-self-child",
+            failingRealloc + linkRows("NULL", "rows[end - 1]"),
+            ["-Drealloc=failing_realloc"],
+          ),
+          [twice, [], twice, twice, twice, twice, noMemory],
+        );
+        // The bottom row's last node takes the root as its left child, a
+        // repeat of a level already walked. These repeats run with growth
+        // failing first, so a scan that missed one would report memory at 64
+        // nodes instead of growing the queue until the run is killed.
+        assert.deepEqual(
+          await actuals(
+            "back-to-root",
+            failingRealloc + linkRows("NULL", "root"),
+            ["-Drealloc=failing_realloc"],
+          ),
+          [twice, [], twice, twice, twice, twice, noMemory],
+        );
+        // With growth working, the full-size tree finds its repeat as well.
+        assert.deepEqual(
+          await actuals("self-child", linkRows("NULL", "rows[end - 1]")),
+          [twice, [], twice, twice, twice, twice, twice],
+        );
+        // The queue grows and then the level ends cannot, so the failure frees
+        // the queue at its new address and the old level ends.
+        const levelEndsFail = `#undef realloc
+void* realloc(void* ptr, size_t size);
+void* failing_realloc(void* ptr, size_t size) {
+    static int calls;
+    return ++calls % 2 ? realloc(ptr, size) : NULL;
+}
+`;
+        assert.deepEqual(
+          await actuals(
+            "level-ends-fail",
+            levelEndsFail + linkRows("NULL", "NULL"),
+            ["-Drealloc=failing_realloc"],
+          ),
+          [
+            ...judge.cases.map((testCase) => testCase.expected),
+            exactFit.expected,
+            noMemory,
+          ],
+        );
+        // In this problem only json_next_tree calls malloc (nextTree uses
+        // calloc), twice per non-empty case: the queue, then the level ends.
+        // Failing every odd call loses only the queue and every even call
+        // only the level ends, and each loss must be reported by itself; an
+        // empty tree never allocates.
+        for (const lost of [1, 0]) {
+          const failingMalloc = `#undef malloc
+void* malloc(size_t size);
+void* failing_malloc(size_t size) {
+    static int calls;
+    return ++calls % 2 == ${lost} ? NULL : malloc(size);
+}
+`;
+          assert.deepEqual(
+            await actuals(
+              `no-malloc-${lost}`,
+              failingMalloc + linkRows("NULL", "NULL"),
+              ["-Dmalloc=failing_malloc"],
+            ),
+            [noMemory, [], noMemory, noMemory, noMemory, noMemory, noMemory],
+          );
+        }
+      } finally {
+        rmSync(dir, { recursive: true, force: true });
+      }
+    },
+  );
+
   it(
     "C Merge k Sorted Lists harness can produce passing Compiler Explorer JSON",
     { skip: noCc },
diff --git a/web/compiler-explorer.js b/web/compiler-explorer.js
index 6a56dcb8..6cb90edf 100644
--- a/web/compiler-explorer.js
+++ b/web/compiler-explorer.js
@@ -403,7 +403,7 @@ static void json_tree(FILE* out, struct TreeNode* root) { if (!root) { fputs("[]
 static void json_random_list(FILE* out, struct Node* node) { struct Node* nodes[10000]; int count = 0; for (struct Node* cur = node; cur && count < 10000; cur = cur->next) { if (isOriginalRandomNode(cur)) { json_string(out, "Returned list must be a deep copy."); return; } nodes[count++] = cur; } fputc('[', out); for (int i = 0; i < count; i++) { if (i) fputc(',', out); int random = -1; for (int j = 0; j < count; j++) if (nodes[i]->random == nodes[j]) random = j; fprintf(out, "[%d,", nodes[i]->val); if (nodes[i]->random && random < 0) { json_string(out, "Returned random pointer must stay inside the copied list."); fputc(']', out); continue; } if (random < 0) fputs("null", out); else fprintf(out, "%d", random); fputc(']', out); } fputc(']', out); }
 static int compare_graph_neighbor_ids(const void* a, const void* b) { int left = *(const int*)a; int right = *(const int*)b; return (left > right) - (left < right); }
 static void json_graph(FILE* out, struct Node* node) { if (!node) { fputs("[]", out); return; } struct Node* nodes[1024]; struct Node* seen[1024] = {0}; int count = 0; nodes[count++] = node; for (int i = 0; i < count && count < 1024; i++) { struct Node* cur = nodes[i]; if (isOriginalGraphNode(cur)) { json_string(out, "Returned graph must be a deep copy."); return; } if (cur->val <= 0 || cur->val >= 1024) { json_string(out, "Returned graph node IDs must be between 1 and 1023."); return; } seen[cur->val] = cur; for (int j = 0; j < cur->numNeighbors; j++) { struct Node* next = cur->neighbors[j]; if (!next) { json_string(out, "Returned graph has a NULL neighbor."); return; } if (next->val <= 0 || next->val >= 1024) { json_string(out, "Returned graph node IDs must be between 1 and 1023."); return; } if (isOriginalGraphNode(next)) { json_string(out, "Returned graph must be a deep copy."); return; } if (seen[next->val] && seen[next->val] != next) { json_string(out, "Returned graph must not contain distinct nodes with the same ID."); return; } if (!seen[next->val]) { seen[next->val] = next; nodes[count++] = next; } } } fputc('[', out); for (int value = 1, printed = 0; value < 1024; value++) { struct Node* cur = NULL; for (int i = 0; i < count; i++) if (nodes[i]->val == value) cur = nodes[i]; if (!cur) continue; if (printed++) fputc(',', out); int degree = cur->numNeighbors; int* neighborIds = NULL; if (degree > 0) { neighborIds = malloc((size_t)degree * sizeof(*neighborIds)); if (!neighborIds) exit(EXIT_FAILURE); for (int j = 0; j < degree; j++) neighborIds[j] = cur->neighbors[j]->val; qsort(neighborIds, (size_t)degree, sizeof(*neighborIds), compare_graph_neighbor_ids); } json_int_array(out, neighborIds, degree); free(neighborIds); } fputc(']', out); }
-static void json_next_tree(FILE* out, struct Node* root) { if (!root) { fputs("[]", out); return; } struct Node* queue[4096]; int head = 0, tail = 0, firstLevel = 1; queue[tail++] = root; fputc('[', out); while (head < tail) { int end = tail; if (!firstLevel) fputc(',', out); firstLevel = 0; fputc('[', out); for (int i = head; i < end; i++) { if (i > head) fputc(',', out); fprintf(out, "%d", queue[i]->val); if (queue[i]->next != (i + 1 < end ? queue[i + 1] : NULL)) { fputs("]", out); fputs("]", out); return; } if (queue[i]->left) queue[tail++] = queue[i]->left; if (queue[i]->right) queue[tail++] = queue[i]->right; } fputc(']', out); head = end; } fputc(']', out); }
+static void json_next_tree(FILE* out, struct Node* root) { if (!root) { fputs("[]", out); return; } int capacity = 64, head = 0, tail = 0, levels = 0; struct Node** queue = malloc(capacity * sizeof(*queue)); int* levelEnd = malloc(capacity * sizeof(*levelEnd)); if (!queue || !levelEnd) { free(queue); free(levelEnd); fputs("null,\\"error\\":", out); json_string(out, "Out of memory while checking next pointers."); return; } queue[tail++] = root; while (head < tail) { int end = tail; for (int i = head; i < end; i++) { if (queue[i]->next != (i + 1 < end ? queue[i + 1] : NULL)) { fputs("null,\\"error\\":", out); json_string(out, "next pointers do not match the tree level order."); free(queue); free(levelEnd); return; } for (int side = 0; side < 2; side++) { struct Node* child = side ? queue[i]->right : queue[i]->left; if (!child) continue; for (int seen = 0; seen < tail; seen++) if (queue[seen] == child) { fputs("null,\\"error\\":", out); json_string(out, "Returned tree must not reach a node twice."); free(queue); free(levelEnd); return; } if (tail == capacity) { struct Node** grownQueue = realloc(queue, 2 * capacity * sizeof(*queue)); if (grownQueue) queue = grownQueue; int* grownEnds = grownQueue ? realloc(levelEnd, 2 * capacity * sizeof(*levelEnd)) : NULL; if (grownEnds) levelEnd = grownEnds; if (!grownQueue || !grownEnds) { free(queue); free(levelEnd); fputs("null,\\"error\\":", out); json_string(out, "Out of memory while checking next pointers."); return; } capacity *= 2; } queue[tail++] = child; } } levelEnd[levels++] = end; head = end; } fputc('[', out); for (int level = 0, start = 0; level < levels; start = levelEnd[level++]) { if (level) fputc(',', out); fputc('[', out); for (int i = start; i < levelEnd[level]; i++) { if (i > start) fputc(',', out); fprintf(out, "%d", queue[i]->val); } fputc(']', out); } fputc(']', out); free(queue); free(levelEnd); }
 static void json_quad_tree_visit(FILE* out, struct Node* node, int* printed) { if (!node) return; if ((*printed)++) fputc(',', out); fprintf(out, "[%d,%d]", node->isLeaf ? 1 : 0, node->isLeaf ? (node->val ? 1 : 0) : 1); if (!node->isLeaf) { json_quad_tree_visit(out, node->topLeft, printed); json_quad_tree_visit(out, node->topRight, printed); json_quad_tree_visit(out, node->bottomLeft, printed); json_quad_tree_visit(out, node->bottomRight, printed); } }
 static void json_quad_tree(FILE* out, struct Node* root) { int printed = 0; fputc('[', out); json_quad_tree_visit(out, root, &printed); fputc(']', out); }
 ${candidateCode}