public VertexIterator adjacentVertices(int v) throws IllegalArgumentException {
        if (v < 0 || v >= numVertices) {
            throw new IllegalArgumentException(
                "Out of range: v = " + v);
        }

        return new AdjacentVerticesIterator(v);
    }
    
    private class AdjacentVerticesIterator(int v) implements VertexIterator {
        private Iterator<Integer> setIterator;
        
        AdjacentVerticesIterator(int v) {
            setIterator = adjacentVertices[v];
    }