CSE202 Greedy algorithms 4.4 Shortest Paths in a Graph shortest path from Princeton CS department to Einstein's house 4.4 Shortest Paths in a Graph shortest path from Princeton CS department to Einstein's house Shortest path tree in Bay area Trees with at most 4 edges G is a tree on n vertices. G is connected with no cycle. G is connected with n-1 edges. G is formed by adding a leaf to a tree of n-1 vertices. There is a unique path between any two vertices. Shortest Path Problem Shortest path network. ! Directed graph G = (V, E). ! Source s, destination t. ! Length !e = length of edge e. Shortest path problem: find shortest directed path from s to t. cost of path = sum of edge costs in path 2 23 3 9 s 18 Cost of path s-2-3-5-t 14 2 6 6 = 9 + 23 + 2 + 16 30 4 19 = 48. 11 5 15 5 6 20 16 t 7 44 45 Dijkstra's Algorithm Dijkstra's algorithm. ! Maintain a set of explored nodes S for which we have determined the shortest path distance d(u) from s to u. ! Initialize S = { s }, d(s) = 0. ! Repeatedly choose unexplored node v which minimizes " (v) = min d(u) + ! e , e = (u,v) : u!S add v to S, and set d(v) = !(v). shortest path to some u in explored part, followed by a single edge (u, v) !e v d(u) u S s 46 Dijkstra's Algorithm Dijkstra's algorithm. ! Maintain a set of explored nodes S for which we have determined the shortest path distance d(u) from s to u. ! Initialize S = { s }, d(s) = 0. ! Repeatedly choose unexplored node v which minimizes " (v) = min d(u) + ! e , e = (u,v) : u!S add v to S, and set d(v) = !(v). shortest path to some u in explored part, followed by a single edge (u, v) !e v d(u) u S s 47 Dijkstra's Algorithm: Proof of Correctness Invariant. For each node u ! S, d(u) is the length of the shortest s-u path. Pf. (by induction on |S|) Base case: |S| = 1 is trivial. Inductive hypothesis: Assume true for |S| = k " 1. ! Let v be next node added to S, and let u-v be the chosen edge. ! The shortest s-u path plus (u, v) is an s-v path of length #(v). ! Consider any s-v path P. We'll see that it's no shorter than #(v). ! Let x-y be the first edge in P that leaves S, P and let P' be the subpath to x. x y ! P is already too long as soon as it leaves S. P' s S u v ! (P) " ! (P') + ! (x,y) " d(x) + ! (x, y) " #(y) " #(v) nonnegative inductive defn of #(y) Dijkstra chose v weights hypothesis instead of y 48 Dijkstra's Algorithm: Implementation For each unexplored node, explicitly maintain !(v) = min d(u) + ! e . e = (u,v) : u" S ! Next node to explore = node with minimum !(v). ! When exploring v, for each incident edge e = (v, w), update !(w) = min { !(w), !(v)+ ! }. e Efficient implementation. Maintain a priority queue of unexplored nodes, prioritized by !(v). Priority Queue PQ Operation Dijkstra Array Binary heap d-way Heap Fib heap † Insert n n log n d log d n 1 ExtractMin n n log n d log d n log n ChangeKey m 1 log n log d n 1 IsEmpty n 1 1 1 1 2 Total n m log n m log m/n n m + n log n † Individual ops are amortized bounds 49 Dijkstra's Shortest Path Algorithm Find shortest path from s to t. 2 24 3 9 s 18 14 2 6 6 30 4 19 11 15 5 5 6 20 16 t 7 44 50 Dijkstra's Shortest Path Algorithm S = { } PQ = { s, 2, 3, 4, 5, 6, 7, t } ! ! 2 24 3 0 9 s 18 14 ! 2 6 6 ! 30 4 19 ! 11 15 5 5 6 20 16 t 7 44 distance label ! ! 51 Dijkstra's Shortest Path Algorithm S = { } PQ = { s, 2, 3, 4, 5, 6, 7, t } delmin ! ! 2 24 3 0 9 s 18 14 ! 2 6 6 ! 30 4 19 ! 11 15 5 5 6 20 16 t 7 44 distance label ! ! 52 Dijkstra's Shortest Path Algorithm S = { s } PQ = { 2, 3, 4, 5, 6, 7, t } decrease key ! X! 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 ! 30 4 19 ! 11 15 5 5 6 20 16 t 7 44 distance label X! 15 ! 53 Dijkstra's Shortest Path Algorithm S = { s } PQ = { 2, 3, 4, 5, 6, 7, t } delmin ! !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 ! 30 4 19 ! 11 15 5 5 6 20 16 t 7 44 distance label X! 15 ! 54 Dijkstra's Shortest Path Algorithm S = { s, 2 } PQ = { 3, 4, 5, 6, 7, t } ! !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 ! 30 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 ! 55 Dijkstra's Shortest Path Algorithm S = { s, 2 } PQ = { 3, 4, 5, 6, 7, t } decrease key X! 33 !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 ! 30 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 ! 56 Dijkstra's Shortest Path Algorithm S = { s, 2 } PQ = { 3, 4, 5, 6, 7, t } X! 33 !X 9 2 24 3 9 0 delmin s 18 14 X 14 ! 2 6 6 ! 30 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 ! 57 Dijkstra's Shortest Path Algorithm S = { s, 2, 6 } PQ = { 3, 4, 5, 7, t } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44 ! 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 ! 58 Dijkstra's Shortest Path Algorithm S = { s, 2, 6 } PQ = { 3, 4, 5, 7, t } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44 ! 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 delmin ! 59 Dijkstra's Shortest Path Algorithm S = { s, 2, 6, 7 } PQ = { 3, 4, 5, t } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35 ! 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 59 !X 60 Dijkstra's Shortest Path Algorithm S = { s, 2, 6, 7 } PQ = { 3, 4, 5, t } delmin 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35 ! 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 59 !X 61 Dijkstra's Shortest Path Algorithm S = { s, 2, 3, 6, 7 } PQ = { 4, 5, t } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35X 34 ! 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 51 59X !X 62 Dijkstra's Shortest Path Algorithm S = { s, 2, 3, 6, 7 } PQ = { 4, 5, t } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35X 34 ! 30 X 4 19 ! 11 15 5 5 6 20 delmin 16 t 7 44 X! 15 51 59X !X 63 Dijkstra's Shortest Path Algorithm S = { s, 2, 3, 5, 6, 7 } PQ = { 4, t } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35X 34 45 !X 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 50 51X 59X !X 64 Dijkstra's Shortest Path Algorithm S = { s, 2, 3, 5, 6, 7 } PQ = { 4, t } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35X 34 45 !X 30 X 4 19 ! 11 15 5 delmin 5 6 20 16 t 7 44 X! 15 50 51X 59X !X 65 Dijkstra's Shortest Path Algorithm S = { s, 2, 3, 4, 5, 6, 7 } PQ = { t } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35X 34 45 !X 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 50 51X 59X !X 66 Dijkstra's Shortest Path Algorithm S = { s, 2, 3, 4, 5, 6, 7 } PQ = { t } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35X 34 45 !X 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 delmin X! 15 50 51X 59X !X 67 Dijkstra's Shortest Path Algorithm S = { s, 2, 3, 4, 5, 6, 7, t } PQ = { } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35X 34 45 !X 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 50 51X 59X !X 68 Dijkstra's Shortest Path Algorithm S = { s, 2, 3, 4, 5, 6, 7, t } PQ = { } 32 X! 33X !X 9 2 24 3 0 9 s 18 14 X 14 ! 2 6 6 44X 35X 34 45 !X 30 X 4 19 ! 11 15 5 5 6 20 16 t 7 44 X! 15 50 51X 59X !X 69 Coin Changing Greed is good.