TheAlgorithms/Java · error · IllegalArgumentException

Invalid node in edge: [{}, {}]

Error message

Invalid node in edge: [{}, {}]

What it means

Thrown by CentroidDecomposition.buildFromEdges when an edge references a node id outside [0, n). The builder indexes adj.get(u) and adj.get(v), so an out-of-range endpoint would throw IndexOutOfBoundsException. The IllegalArgumentException reports both offending endpoints.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/trees/CentroidDecomposition.java:208

        }
        if (edges.length != n - 1) {
            throw new IllegalArgumentException("Tree must have exactly n-1 edges");
        }

        List<List<Integer>> adj = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            adj.add(new ArrayList<>());
        }

        for (int[] edge : edges) {
            if (edge.length != 2) {
                throw new IllegalArgumentException("Each edge must have exactly 2 nodes");
            }
            int u = edge[0];
            int v = edge[1];

            if (u < 0 || u >= n || v < 0 || v >= n) {
                throw new IllegalArgumentException("Invalid node in edge: [" + u + ", " + v + "]");
            }

            adj.get(u).add(v);
            adj.get(v).add(u);
        }

        return new CentroidTree(adj);
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Convert 1-based external ids to 0-based by subtracting 1 before building edges.
2. Set n to max(edge ids) + 1 (after conversion) so all endpoints are in range.
3. Validate every endpoint against [0, n-1] at the input boundary.
4. Reject self-loops and duplicate edges which often accompany id bugs.

Example fix

// before
CentroidTree ct = CentroidDecomposition.buildFromEdges(n, edges);
// after
for (int[] e : edges) {
    if (e[0] < 0 || e[0] >= n || e[1] < 0 || e[1] >= n) {
        throw new IllegalArgumentException("edge out of range: " + Arrays.toString(e));
    }
}
CentroidTree ct = CentroidDecomposition.buildFromEdges(n, edges);

for (int[] e : edges) {
    if (e[0] < 0 || e[0] >= n || e[1] < 0 || e[1] >= n) {
        throw new IllegalArgumentException("edge endpoint out of range: " + Arrays.toString(e));
    }
}

When it happens

Trigger: Passing edges with 1-based node ids into a 0-based builder. Passing an edge [u, n] where n equals the node count (valid range is 0..n-1). Negative ids from sentinel/uninitialized values.

Common situations: Indexing-convention mismatch between input data (often 1-based) and the algorithm (0-based). Node count n set too low relative to the actual max id in edges. Edges built from a different vertex labeling than expected.

Related errors

• Invalid node: {}
• Number of nodes must be positive
• Tree must have exactly n-1 edges
• Each edge must have exactly 2 nodes
• Tree cannot be empty or null

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/0ef1e199d9262906. Report an issue: GitHub.