/**
 * Represents a graph data structure using an adjacency list.
 */
export declare class Graph {
    private adjacencyList;
    /**
     * Adds a vertex to the graph.
     * @param {string} vertex - The vertex to add.
     */
    addVertex(vertex: string): void;
    /**
     * Adds an edge between two vertices.
     * @param {string} vertex1 - The first vertex.
     * @param {string} vertex2 - The second vertex.
     */
    addEdge(vertex1: string, vertex2: string): void;
    /**
     * Performs a breadth-first search (BFS) starting from a given vertex.
     * @param {string} start - The starting vertex.
     * @returns {string[]} The vertices visited in BFS order.
     */
    bfs(start: string): string[];
}