# Discrete Mathematics Questions and Answers – Spanning Trees

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This set of Discrete Mathematics Multiple Choice Questions & Answers (MCQs) focuses on “Spanning Trees”.

1. Spanning trees have a special class of depth-first search trees named _________
a) Euclidean minimum spanning trees
b) Tremaux trees
c) Complete bipartite graphs
d) Decision trees

Explanation: A tremaux tree of an undirected graph G is a spanning tree of G which is rooted at one of its vertices with the property that every two adjacent vertices in G are related to each other as an ancestor and descendant in the tree.

2. If the weight of an edge e of cycle C in a graph is larger than the individual weights of all other edges of C, then that edge ________
a) belongs to an minimum spanning tree
b) cannot belong to an minimum spanning tree
c) belongs to all MSTs of the graph
d) can not belong to the graph

Explanation: For any cycle C in the graph, if the weight of an edge e of C is larger than the individual weights of all other edges of C, then this edge cannot belong to an MST.

3. For every spanning tree with n vertices and n edges what is the least number of different Spanning trees can be formed?
a) 2
b) 5
c) 3
d) 4

Explanation: If graph is connected and has ‘n’ edges, there will be exactly one cycle, if n vertices are there. A different spanning tree can be constructed by removing one edge from the cycle, one at a time. The minimum cycle length can be 3. So, there must be at least 3 spanning trees in any such Graph. Consider a Graph with n = 4, then 3 spanning trees possible at maximum (removing edges of cycle one at a time, alternatively). So, any Graph with minimum cycle length ‘3’ will have at least 3 spanning trees.

4. Time complexity of Prim’s algorithm is _________
a) O((V+E)logV)
b) O(E+V)
c) O(E)
d) O(V+1)

Explanation: In Prim’s Algorithm, we will start with an arbitrary node (take any point to start) and mark it. In each iteration, a new vertex is marked that is adjacent to the one that we have already marked. Each vertex is inserted in the priority queue only once and insertion in priority queue take logarithmic time. Hence, the time complexity of Prim’s Algorithm is O((V+E)logV).

5. What is the time complexity of Kruskal’s algorithm?
a) O(ElogV)
b) O(V+logE)
c) O(E+1)
d) O(V2)

Explanation: In Kruskal’s algorithm, at each iteration, we will select the edge with the lowest weight. So, we will start with the lowest weighted edge first. After that we will select the second lowest weighted edge. In Kruskal’s algorithm, most time consuming operation is sorting because the total complexity of the Disjoint-Set operations will be O(ElogV) and it is the overall Time Complexity of the algorithm.

6. An immediate application of minimum spanning tree ______
a) gesture analysis
b) handwriting recognition
c) fingerprint detection
d) soft computing

Explanation: Minimum spanning tree is the spanning tree where the cost is minimum among all the spanning trees. It is used in network designing, in the algorithms predicting the travelling salesman problem,multi-terminal minimum cut problem and minimum-cost weighted perfect matching. It can also used in Handwriting recognition and image segmentation.

6. If minimum cost edge of a graph is unique, then that edge will be added to any MST. Choose the correct option.
a) false
b) maximum cost edge is added
c) true
d) minimum cost edge need not be unique

Explanation: If the edge was not included in the MST, removing any of the (larger cost) edges in the cycle formed after adding e to the MST, would yield a spanning tree of smaller weight. Thus, if the minimum cost edge e of a graph is unique, then this edge is included in any MST.

7. A complete undirected graph of n nodes can have maximum ______ spanning trees.
a) nn+1
b) nn-2
c) $$\frac{n(n+1)}{2}$$
d) n

Explanation: The spanning tree does not contain any cycle. If a spanning tree has n nodes, there are n-1 edges. A complete graph can have a maximum of nn-2 number of spanning trees.

8. The spanning tree will be maximally acyclic if ____________
a) one additional edge makes a cycle in the tree
b) two additional edges makes a cycle in the tree
c) removing one edge makes the tree cycle free
d) removing two edges make the tree cycle free

Explanation: A connected graph G can have more than one spanning tree. Removing one edge from the spanning tree will make the graph disconnected and the spanning tree is minimally connected. Adding one edge to the spanning tree will create a circuit or loop and the spanning tree is maximally acyclic.

9. In a maximum spanning tree the weighted graph is of _______
a) maximum number of edges
b) maximum number of cyclic trees
c) minimum number of vertices
d) maximum weight

Explanation: A maximum spanning tree can be computed by negating the weights for each edge and applying Kruskal’s algorithm. Thus, it is a spanning tree of a weighted graph having maximum weight assigned to all the edges.

10. Prim’s algorithm can be implemented using _______
a) a stack data structure
c) priority queue data structure
d) bubble sort