# VE281 Programming Assignment Five: Graph Algorithms solved

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## Description

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I. Motivation
1. To give you experience in implementing a graph data structure using the adjacency list
representation.
2. To give you experience in implementing a few graph algorithms.
II. Programming Assignment
You will read from the standard input a description of a directed graph and then report two
things on that graph:
1. Whether the graph is a directed acyclic graph (DAG).
2. Determine the total weight of a minimum spanning tree (MST) of the graph when it is treated
as an undirected graph.
1. Input Format
The first line in the input specifies the number of nodes in the graph, 𝑁. The nodes in the graph
are indexed from 0 to 𝑁 − 1. Each subsequent line represents a directed edge by 3 numbers in
the form:

where both and are integers in the range [0, 𝑁 − 1] ,
representing the start node and the end node of the edge, respectively, and is an
integer representing the edge weight.
An example of input is
4
0 1 5
1 2 3
2 3 4
3 1 6
It represents the following directed graph:
0 1
2
3
5
3
4
6
Typically, we will describe the graph in a file. However, since your program takes input from the
standard input, you need to use the Linux input redirection “<” on the command line to read the
graph from the file.
2. Output Specification
Your program writes to the standard output. It will first tell whether the graph is a DAG or not.
Then, it will calculate the total weight of a MST of the original graph when treated as an
undirected graph.
a) Telling whether the graph is a DAG or not
If the graph is a DAG, your program should print:
The graph is a DAG
Otherwise, print:
The graph is not a DAG
b) Calculating the total weight of an MST
You should treat all the directed edges in the original graph as undirected edges, and obtain the
total edge weight of a minimum spanning tree for the undirected graph. If there exists a spanning
tree, your program should print the total edge weight of an MST. Specifically, it should print the
following line:
The total weight of MST is
where specifies the total edge weight of an MST.
If there exists no spanning tree, your program should print:
No MST exists!
For the example graph shown above, the valid output should be:
The graph is not a DAG
The total weight of MST is 12
III. Program Arguments and Error Checking
Your program takes no arguments. You do not need to do any error checking. You can assume
that all the inputs are syntactically correct.
IV. Implementation Requirements and Restrictions
 You must implement the graph data structure using the adjacency list representation.
 You must make sure that your code compiles successfully on a Linux operating system. You
are required to write your own Makefile and submit it together with your source code files.
Your compiled program should be named as main exactly.
 We highly encourage the use of the STL, such as vector, deque, map, etc., for this
project, with the exception of two prohibited features: The C++11 regular expressions library
(whose implementation in gcc 4.7 is unreliable) and the thread/atomics libraries (which spoil
runtime measurements). Do not use other advanced libraries (e.g., boost, pthreads, etc).
 Output should only be done where it is specified.
V. Hints
 You may want to define the following three abstract data types: node, edge, and graph.
 To improve the performance of your program, we recommend you to add -O2 option when
 For large test cases, I/O could be the runtime bottleneck. To reduce the I/O time, we
recommend you to put the following two statements at the beginning of your main function:
std::ios::sync_with_stdio(false);
std::cin.tie(0);
VI. Testing
We have supplied three input files g1.in, g2.in, and g3.in for you to test your program.
The outputs of our program for these three inputs are also provided. They are g1.out, g2.out,
and g3.out. All these files are located in the Programming-Assignment-Five-Related-Files.zip.
To do the test, type a similar command to the following into the Linux terminal once your
program has been compiled:
./main < g1.in > test.out
diff test.out g1.out
If the diff program reports any differences at all, you have a bug.
These are the minimal amount of tests you should run to check your program. Those programs
that do not pass these tests are not likely to receive much credit. You should also write other
different test cases yourself to test your program extensively.
VII. Submitting and Due Date
You should submit all the related .h files, .cpp files, and the Makefile via the online
judgement system. The Makefile compiles a program named main. Please also include a pdf
file containing all of your source code. See announcement from the TAs for the details about
how to submit these files. The submission deadline is 11:59 pm on Dec. 13, 2018.
1. Functional Correctness
2. Implementation Constraints
3. General Style
4. Performance
Functional Correctness is determined by running a variety of test cases against your program,
checking your solution using our automatic testing program. We will grade Implementation
Constraints to see if you have met all of the implementation requirements and restrictions. For
example, we will check whether you implement the graph data structure using the adjacency list
representation. General Style refers to the ease with which TAs can read and understand your
program, and the cleanliness and elegance of your code. For example, if it is hard for the TAs to