The BFS is performed on the above undigraph with the vertex s as the starting point. The neighbors of the same vertex are in the order of a~z. When the vertex c is just out of the queue, the vertices in the queue from the head of the queue to the end of the queue are: 以顶点s为起点对以上无向图进行BFS，同一顶点的邻居之间以a~z为顺序，顶点c刚出队时队列中顶点从队头到队尾为：
Ad, e
Be, d
Cd, a
Da, d