21. A Hamiltonian path in a graph is a tour of the vertices of the graph that visits each vertex once and only once and starts and ends at different vertices.
For each of the graphs shown in Exercise 17, does the graph have a Hamiltonian path?
Does each of these graphs have a Hamiltonian path that starts at X1 and ends at X2?
Describe three real-world situations where finding a Hamiltonian path in a graph would be required.