public String toString() {
    	StringBuilder sb = new StringBuilder();
        sb.append("Vertices: " + numVertices + " " + "Edges: {");
        
        for (int i = 0; i < numVertices; i++) {
        	for (int j : adjacentVertices[i]) {
                if (j >= i) {                
                	if (!edgeCosts.containsKey(new Edge(i, j))) {
                		sb.append("(" + i + "," + j + "), ");
                	} else {
                		sb.append("(" + i + "," + j + "," + edgeCosts.get(new Edge(i, j)) + "), ");
                	}
                }
            }
        }
        
        if (numEdges > 0) {
            sb.setLength(sb.length() - 2); // Remove trailing ", "
        }
        
        sb.append("}");
        
        return sb.toString();
    }