krahets/hello-algo · error · Error

Illegal Argument Exception

Error message

Illegal Argument Exception

What it means

Thrown by GraphAdjList.addEdge(vet1, vet2) when either vertex is absent from the adjacency list, or when vet1 === vet2 (self-loop). The method then pushes each vertex into the other's neighbor list; the guard prevents pushing into an undefined neighbor array.

Source

Thrown at zh-hant/codes/typescript/chapter_graph/graph_adjacency_list.ts:37

            this.addVertex(edge[0]);
            this.addVertex(edge[1]);
            this.addEdge(edge[0], edge[1]);
        }
    }

    /* 獲取頂點數量 */
    size(): number {
        return this.adjList.size;
    }

    /* 新增邊 */
    addEdge(vet1: Vertex, vet2: Vertex): void {
        if (
            !this.adjList.has(vet1) ||
            !this.adjList.has(vet2) ||
            vet1 === vet2
        ) {
            throw new Error('Illegal Argument Exception');
        }
        // 新增邊 vet1 - vet2
        this.adjList.get(vet1).push(vet2);
        this.adjList.get(vet2).push(vet1);
    }

    /* 刪除邊 */
    removeEdge(vet1: Vertex, vet2: Vertex): void {
        if (
            !this.adjList.has(vet1) ||
            !this.adjList.has(vet2) ||
            vet1 === vet2 ||
            this.adjList.get(vet1).indexOf(vet2) === -1
        ) {
            throw new Error('Illegal Argument Exception');
        }
        // 刪除邊 vet1 - vet2
        this.adjList.get(vet1).splice(this.adjList.get(vet1).indexOf(vet2), 1);

View on GitHub (pinned to 69932aed18)

Solutions

1. Call addVertex for both endpoints before addEdge.
2. Ensure vet1 and vet2 are distinct references.
3. Verify graph.adjList.has(vet1) && graph.adjList.has(vet2) before addEdge.

Example fix

// before
graph.addEdge(v1, v2); // throws if vertex not registered or v1 === v2

// after
graph.addVertex(v1);
graph.addVertex(v2);
if (v1 !== v2) {
    graph.addEdge(v1, v2);
}

// Register both vertices and ensure distinct before adding edge
graph.addVertex(v1);
graph.addVertex(v2);
if (v1 !== v2) {
    graph.addEdge(v1, v2);
}

function canAddEdge(graph: GraphAdjList, v1: Vertex, v2: Vertex): boolean {
    return graph.adjList.has(v1) && graph.adjList.has(v2) && v1 !== v2;
}

try {
    graph.addEdge(v1, v2);
} catch (e) {
    if (e.message === 'Illegal Argument Exception') {
        // vertex not registered or self-loop — handle
    } else throw e;
}

When it happens

Trigger: Calling addEdge with a vertex not added via addVertex, or passing the same Vertex reference for both endpoints.

Common situations: Forgetting to addVertex before connecting edges; using Vertex objects from a different graph; attempting self-loops in an undirected simple graph.

Related errors

• Illegal Argument Exception
• Illegal Argument Exception
• Illegal Argument Exception
• Illegal Argument Exception
• Index Out Of Bounds Exception

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/4a3b3dffcec012b2. Report an issue: GitHub.