A. head (tail (head (L)))                        B. head (head(head(L)))

C. tail (head (tail (L)))                           D. head (head (tail (L)))

7.已知一棵含50个结点的二叉树中只有一个叶子结点，则该树中度为1的结点个数为（    ）

A. 0                                                    B. 1

C. 48                                                  D. 49

8.在一个具有n个顶点的有向图中，所有顶点的出度之和为D_out ，则所有顶点的入度之和为（    ）

A. D_out                                                B. D_out-1

C. D_out+1                                            D. n

9.如图所示的有向无环图可以得到的拓扑序列的个数是（    ）

A. 3                                                    B. 4

C. 5                                                    D. 6

10.如图所示的带权无向图的最小生成树的权为（    ）

A. 51                                                  B. 52

C. 54                                &