HW2_April2018_solutions.pdf

# Describe an algorithm to do this solution we can

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paths, we also want to know how many shortest paths there are. Describe an algorithm to do this. Solution: We can achieve this easily by modifying the Dijkstra Algorithm procedure ModDijkstra ( E, V, s ) Create priority queue Q for v V do dist [ v ] ← ∞ , count [ v ] 1 add v into Q end for dist [ s ] 0 while Q 6 = do u argmin u 0 ( dist [ u 0 ]) remove u from Q for v s.t. ( u, v ) E do τ dist [ u ] + length ( u, v ) if τ < dist [ v ] then dist [ v ] τ , count [ v ] count [ u ] . reset counting end if if τ = dist [ v ] then count [ v ] = count [ v ] + count [ u ] end if end for end while end procedure 3

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5. Suppose that you are given a directed graph with non-negative edge weights, and a target node t . You want to find the shortest path to target node t from every other node. (Notice that this is different from what we covered in class: in class, we considered the problem of finding the shortest path from the specified node to every other node.) Describe an algorithm to calculate these values. Solution: Apply Dijistra’s algorithm on the reversed graph G R and use target t as source 4
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