6.006 Lecture 17: Bellman-Ford

Lecture 17 Shortest Paths III: Bellman-Ford 6.006 Fall 2011 Lecture 17: Shortest Paths III: Bellman-Ford Lecture Overview • Review: Notation • Generic S.P. Algorithm • Bellman-Ford Algorithm – Analysis – Correctness Recall: path p = < v0 , v1 , . . . , vk > (v1 , vi+1 ) ∈ E 0 ≤ i < k k −1 X w(p) = w(vi , vi+1 ) i−0 Shortest path weight from u to v is δ(u, v). δ(u, v) is ∞ if v is unreachable from u, undefined if there is a negative cycle on some path from u to v. -ve u v Figure 1: Negative Cycle. 1 Lecture 17 Shortest Paths III: Bellman-Ford 6.006 Fall 2011 Generic S.P. Algorithm for v ∈ V : Initialize: d [v] Π [v] ← ← ∞ NIL d[S] ← 0 repeat select edge (u, v) [somehow]  if d[v] > d[u] + w(u, v) :  d[v] ← d[u] + w(u, v) π[v] ← u until you can’t relax any more edges or you’re tired or . . . Main: “Relax” edge (u, v) Complexity: Termination: Algorithm will continually relax edges when there are negative cycles present. 1 u 0 d[u] 1 1 0 -1 -2 -1 -4 2 etc 3 2 1 0 v 1 4 Figure 2: Algorithm may not terminate due to negative cycles. Complexity could be exponential time with poor choice of edges. 2 Lecture 17 Shortest Paths III: Bellman-Ford v0 T(n) = 3 + 2T(n-2) T(n) = θ(2n/2) 6.006 Fall 2011 v1 v2 v3 v4 v5 v6 4 8 10 12 13 10 11 8 9 14 13 12 11 10 ORDER (v0, v1) (v1, v2) all of v2, vn (v0, v2) 4 6 all of v2, vn Figure 3: Algorithm could take exponential time. The outgoing edges from v0 and v1 have weight 4, the outgoing edges from v2 and v3 have weight 2, the outgoing edges from v4 and v5 have weight 1. 5-Minute 6.006 Figure 4 is what I want you to remember from 6.006 five years after you graduate! Bellman-Ford(G,W,s) Initialize () for i = 1 to |V | − 1 for each edge (u, v) ∈ E: Relax(u, v) for each edge (u, v) ∈ E do if d[v] > d[u] + w(u, v) then report a negative-weight cycle exists At the end, d[v] = δ(s, v), if no negative-weight cycles. Theorem: If G = (V, E) contains no negative weight cycles, then after Bellman-Ford executes d[v] = δ(s, v) for all v ∈ V . 3 Lecture 17 Shortest Paths III: Bellman-Ford Exponential Bad Polynomial Good T(n) = C1 + C2T(n - C3) T(n) = C1 + C2T(n / C3) if C2 > 1, trouble! Divide & Explode 6.006 Fall 2011 C2 > 1 okay provided C3 > 1 if C3 > 1 Divide & Conquer Figure 4: Exponential vs. Polynomial. Proof: Let v ∈ V be any vertex. Consider path p = hv0 , v1 , . . . , vk i from v0 = s to vk = v that is a shortest path with minimum number of edges. No negative weight cycles =⇒ p is simple =⇒ k ≤ |V | − 1. Consider Figure 6. Initially d[v0 ] = 0 = δ(s, v0 ) and is unchanged since no negative cycles. After 1 pass through E, we have d[v1 ] = δ(s, v1 ), because we will relax the edge (v0 , v1 ) in the pass, and we can’t find a shorter path than this shortest path. (Note that we are invoking optimal substructure and the safeness lemma from Lecture 16 here.) After 2 passes through E, we have d[v2 ] = δ(s, v2 ), because in the second pass we will relax the edge (v1 , v2 ). After i passes through E, we have d[vi ] = δ(s, vi ). After k ≤ |V | − 1 passes through E, we have d[vk ] = d[v] = δ(s, v).  Corollary If a value d[v] fails to converge after |V | − 1 passes, there exists a negative-weight cycle reachable from s. Proof: After |V | − 1 passes, if we find an edge that can be relaxed, it means that the current shortest path from s to some vertex is not simple and vertices are repeated. Since this cyclic path has less weight than any simple path the cycle has to be a negative-weight cycle.  4 Lecture 17 Shortest Paths III: Bellman-Ford 6.006 Fall 2011 1 -1 ∞ -1 B -1 0 4 4 2 2 3 3 5 E 2 1 8 2 6 C 5 ∞ B -1 2 3 7 4 A 1 0 ∞ 4 -3 3 5 E 2 1 8 2 D C ∞ 2 End of pass 1 2 3 7 4 A 1 6 5 D ∞ ∞ 1 1 -3 1 -2 2 3 End of pass 2 (and 3 and 4) Figure 5: The numbers in circles indicate the order in which the δ values are computed. v v1 vk δ (s, vi) = δ (s, vi-1) + w (vi-1,vi) S p: v0 v2 Figure 6: Illustration for proof. Longest Simple Path and Shortest Simple Path Finding the longest simple path in a graph with non-negative edge weights is an NPhard problem, for which no known polynomial-time algorithm exists. Suppose one simply negates each of the edge weights and runs Bellman-Ford to compute shortest paths. Bellman-Ford will not necessarily compute the longest paths in the original graph, since there might be a negative-weight cycle reachable from the source, and the algorithm will abort. Similarly, if we have a graph with negative cycles, and we wish to find the longest simple path from the source s to a vertex v, we cannot use Bellman-Ford. The shortest simple path problem is also NP-hard. 5