1. CS420 lecture twelve Shortest Paths wim bohm cs csu

2. Shortest Paths Problems Given a weighted directed graph G=(V,E) find the shortest path – path length is the sum of its edge weights. The shortest path (SP) from u to v is  if there is no path from u to v.

3. Variations 1) SSSP (Single source SP): find the SP from some node s to all nodes in the graph. 2) SPSP (single pair SP): find the SP from some u to some v. We can use 1) to solve 2), also there is no asymptotically faster algorithm for 2) than that for 1).

4. Variations 3) SDSP (single destination SP) can use 1) by reversing its edges. 4) APSP (all pair SPs) could be solved by |V| applications of 1), but can be solved faster. We will thus concentrate on SSSP and APSP.

5. Optimal Substructure the property that the SP from u to v via w consists of a SP from u to w, and a SP from w to v or that a sub path from p to q is a SP from p to q.

6. Negative weight edges and cycles Negative weight cycles create an ill defined problem. Why? If some weights in the graph are negative but no cycle in the graph has a negative length, the SP problem stays well defined. Some algorithms (Dijkstra) assume all weights to be positive, some (Bellman Ford) allow negative weights but not negative length cycles.

7. Zero length cycles We have already ruled out negative length cycles, a shortest path cannot contain a positive length cycle, what about zero length cycles? We can remove zero length cycles and produce a path with the same length. Hence we can assume that shortest paths have no cycles and thus have at most |V|-1 edges.

8. SSSP and Shortest path trees For each vertex v on the shortest path (or the shortest path in construction) we maintain its predecessor  (v), a node or nil. The predecessor sub graph G  = (V , E  ) has the vertices v and edges  (v)  nil plus the source s. Shortest path algorithms make G  a shortest path tree with root s with shortest paths to all vertices reachable from s.

9. d(v) For each vertex v we maintain d(v), the shortest-path estimate: an upper bound on the shortest path length to v. We initialize d-s and  -s as follows: Init-SingleSource (G,s){ for each vertex v {d[v]=  ;  [v]=nil} d[s]=0; }

10. ∞ Arithmetic with  : if a  - , we have a+  =  +a =  if a  , a+(-  )=(-  )+a=- .

11. Relax Shortest paths algorithms use the operation relax edge (u,v): testing whether we can improve a path from s to v by using edge (u,v) and, if so, updating d[v] and  [v]: Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} }

12.  (s,v) The final value for d[v] is denoted as  (s,v), the shortest path from s to v. Assuming the algorithm starts with Init-SingleSource and only does updates on d and  using Relax, the following properties of shortest paths and relax can be proved (Cormen et. al., ch. 24.5).

13. Properties Triangle Inequality: For any edge (u,v):  (s,v) <=  (s,u) + w(u,v) optimal sub structure Upper-bound Property: d[v] >=  (s,v) and once d[v] achieves  (s,v), it does not change anymore. nature of relax

14. More Properties No-path property: If there is no path from s to v, d[v]=  (s,v)=  relax never changed d Convergence property: If s  u  v is a shortest path and d[u]=  (s,u) before relaxing edge (u,v), then d[v] =  (s,v) afterward. optimal sub structure

15. and more Path-relaxation property: If p = is a shortest path from s=v 0 to v k, and the edges are relaxed in order (v 0,v 1 ), (v 1,v 2 ) etc., then d[v]=  (s,v). inductive argument Predecessor-subgraph property: Once d[v]=  (s,v) for all v in V, the predecessor subgraph is the shortest path tree rooted at s. nature of Relax

16. Bellman-Ford SSSP Allows negative edge weights Checks for negative length cycles returns false if there is a negative length cycle otherwise, BF returns true and and the shortest paths in d and the shortest path tree in .

17. Bellman-Ford(G,w,s){ Init-SingleSource(G,s) for i = 1 to |V|-1 for each (u,v) in E Relax(u,v,w) // check for negative cycles for each (u,v) in E if (d[v]> d[u]+w(u,v) return false return true } complexity?

18. Bellman-Ford(G,w,s){ Init-SS(G,s) for i = 1 to |V|-1 for each (u,v) in E Relax(u,v,w) // check for negative cycles for each (u,v) in E if (d[v]> d[u]+w(u,v) return false return true } complexity O(|V| * |E| )

19. Correctness Bellman-Ford Cormen et.al.: proof in the case that there are no negative length cycles is based on path relaxation lemma: for each shortest path the first edge is found in sweep one, the i-th edge is found in sweep i. If after |V|-1 sweeps there is still improvement then there must be a negative length path.

20. 0 s ∞ b ∞ c ∞ a 6 7-3 5 -2 Example BF1. State: right after Init-SS. The nodes contain their d values, all  values are nil. BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). What happens in the first sweep?..... Relax all edges.....

21. 0 s ∞ b ∞ c ∞ a 6 7-3 5 -2 Example BF1. State: right after Init-SS. The nodes contain their d values, all  values are nil. BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} } What happens in the first sweep? In the first sweep: d[b] > 0+6 so d[b]=6 and  [b]=s d[c] > 0+7 so d[c]=7 and  [c]=s

22. 0 s 6 b 7 c ∞ a 6 7-3 5 -2 Example BF1. State: after sweep 1 BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} } What happens in the second sweep?

23. 0 s 6 b 7 c ∞ a 6 7-3 5 -2 Example BF1. State: after sweep 1 BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} } What happens in the second sweep? In the second sweep: d[a] > 6+5 so d[a]=11 and  [a]=b d[a] > 7-3 so d[a]=4 and  [a]=c d[b] > 4-2 so d[b]=2 and  [b]=a

24. 0 s 2 b 7 c 4 a 6 7-3 5 -2 What happens in the third sweep? What is returned?

25. 0 s 2 b 7 c 4 a 6 7-3 5 -2 In the third sweep nothing changes BF1 returns true

26. 0 s ∞ b ∞ c ∞ a 6 7-3 -5 2 Example BF2. Slightly changed state: right after Init-SS. The nodes contain their d values, all  values are nil. BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} } What happens in the first sweep?

27. 0 s 6 b 7 c ∞ a 6 7-3 -5 2 Example BF2. State after first sweep BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} } In the first sweep: d[b] > 0+6 so d[b]=6 and  [b]=s d[c] > 0+7 so d[c]=7 and  [c]=s

28. 0 s 6 b 7 c ∞ a 6 7-3 -5 2 Example BF2. State after first sweep BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} } What happens in the second sweep?

29. 0 s 6 b 7 c 1 a 6 7-3 -5 2 Example BF2. State after second sweep BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} } In the second sweep: d[a] > 6-5 so d[a]=1 and  [a]=b

30. 0 s 6 b 7 c 1 a 6 7-3 -5 2 Example BF2. State after second sweep BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} } What happens in the third sweep?

31. 0 s 3 b 7 c 1 a 6 7-3 -5 2 Example BF2. State after third sweep BF does 3 sweeps over all edges, assume order (a,b), (b,a), (c,a), (s,b), (s,c). Relax(u,v,w) { if (d[v] > (r = d[u]+w(u,v))) {d[v] = r;  [v]= u} } In sweep three we can now improve d[b] but the predecessor sub graph is broken!! We now can keep lowering d[a] and and d[b] in subsequent sweeps. The Bellman-Ford algorithm detects this in its last sweep and returns false.

32. SSSP in a DAG By relaxing edges in a DAG in topological sort order we need to only relax each edge once because in any path p =, (v 0,v 1 ) topologically precedes (v 1,v 2 ) etc. So the path relaxation property implies d[v]=  (s,v) for all v.

33. Dijkstra's SSSP Dijkstra's greedy algorithm solves SSSP for directed graphs with non-negative edges. Very much like in Primm's minimal spanning tree algorithm, a set S of vertices whose minimal path has been determined is stepwise extended, and distances to yet unreached vertices are updated. The vertices are maintained in a min priority queue, keyed on their d value.

34. Back to Primm MST In lecture 8 Greedy algorithms we sketched Primm's MST: – the growing MST A is a tree – start with arbitrary point, pick minimal emanating edge – keep adding minimal distance to A nodes, adjusting the minimal distance of the points not in to A – store V in min priority Queue

35. MST-Prim(G,w,s) // G=(V,E), weights w, root s init_SingleSource(G,s) Q=V; // in initial priority Queue form, // eg heapified while Q not empty u = extract-min(Q) // first one will be s for each v in adj(u) if v in Q and w(u,v) < u.d v.d = w(u,v) ; v.  = u

36. Complexity Primm Q: binary min heap (see sorting lecture) – we can build the heap in O(|V|) time The while loop executes |V| times – each extract-min takes O(lg|V|) time totalling O(|V|lg|V|) time

37. Complexity Primm cont' The inner for loop executes |E| times in total – because each vertex is extracted only once and therefore each edge in the adjacency list representing G is considered once – v.d = w(u,v) in the conditional in the innermost loop involves a decrease-key operation in the heap taking O(lg|V|) time Hence, complexity when using priority min heap: O((|V|+|E|)(lg(|V|)) which is O(|E|lg|V|)

38. Dijkstra(G,w,s) { // very much like Primm S = empty; Q = V(G); // min priority queue while (Q not empty) { u=Extract-min(Q); S=S  {u} for each adjacent v of u Relax(u,v,w) } The while loop keeps Q=V-S invariant The first u=s The Relax function decreases d-values for some vertices using the decrease-key min-priority queue function (see Sorting lecture). Relax creates d and 

39. Dijkstra(G,w,s) { S = empty; Q = V(G); // min priority queue while (Q not empty) { u=Extract-min(Q); S=S  {u} for each adjacent v of u Relax(u,v,w) } Correctness proof very similar to Primm's MST proof

40. 0 s ∞ b ∞ c ∞ a 2 7 1 5 2 1

41. 0 s ∞ b ∞ c ∞ a 2 7 1 5 2 1 0 s 2 b 7 c ∞ a 2 7 1 5 2 1

42. 0 s ∞ b ∞ c ∞ a 2 7 1 5 2 1 0 s 2 b 7 c ∞ a 2 7 1 5 2 1 0 s 2 b 3 c 7 a 2 7 1 5 2 1

43. 0 s ∞ b ∞ c ∞ a 2 7 1 5 2 1 0 s 2 b 7 c ∞ a 2 7 1 5 2 1 0 s 2 b 3 c 7 a 2 7 1 5 2 1 0 s 2 b 3 c 4 a 2 7 1 5 2 1

44. Floyd-Warshall's APSP algorithm Floyd-Warshall's dynamic programming APSP algorithm runs in  (|V| 3 ) time and uses the adjacency matrix representation: w ij = 0 if i=j weight ij if (i,j) in E  otherwise Negative weight edges can be present, but negative length cycles cannot.

45. vertex sets Denote the Vertex set V to be {1,2,...,n} and let V k be the subset {1,2,...k}. The algorithm uses a recurrence, which in step k considers paths from all i to all j either (don't take node k into account) with intermediate nodes from V k-1 only or (do take node k into account) with intermediate nodes V k-1 and also node k

46. k intermediate node in V k ? If k is not an intermediate node then the shortest path from i to j in step k is equal to the shortest path in step k-1. If k is an intermediate node then the shortest path from i to j in step k is the shortest from i to k in step k-1 plus the shortest path from k to j in step k-1.

47. V k :via set via which paths from i to j go equation form of previous slide: d ij (k) = w ij if k=0 min(d ij (k-1), d ik (k-1) + d kj (k-1) ) if k>0

48. Floyd Warshall Floyd-Warshall(W) D=W for k = 1 to n for i = 1 to n for j = 1 to n D[i,j]=min(D[i,j], D[i,k]+D[k,j]

49. Predecessors in Floyd-Warshall A recurrence for the predecessor  can be derived much like the recurrence for d.  ij (0) = nil if i=j or (i,j) not in E i otherwise  ij (k) =  ij (k-1) if d ij (k-1)  d ik (k-1) + d kj (k-1)  kj (k-1) if d ij (k-1) > d ik (k-1) + d kj (k-1) Code for computing the predecessor is easily incorporated in the Floyd-Warshall function.

50. transitive closure Given a graph G=(V,E), the transitive closure graph G * =(V,E * ) has an edge (i,j) if there is a path from i to j in G. G * can be computed using Floyd-Warshall by – initially assigning D ij =1 if (i,j) in E and 0 otherwise, – and replacing min by or and + by and in the loop.