# CS 10: Problem solving via Object Oriented Programming Winter 2017

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1 CS 10: Problem solving via Object Oriented Programming Winter 2017 Tim Pierson 260 (255) Sudikoff Day 17 Shortest Path

2 Agenda 1. Shortest- path simulalon 2. Dijkstra s algorithm 3. A* search 4. Implicit graphs 2

3 Previously we looked at finding the minimum number of steps between nodes Breadth First Search B F H BFS is a good choice A C Can find shortest number of steps between source and any other node Start D E G I Could use BFS on a map to plan driving routes between ciles Goal 3

4 Previously we looked at finding the minimum number of steps between nodes Breadth First Search 2 1 A 3 4 B C 6 7 F 8 9 H BFS is a good choice Can find shortest number of steps between source and any other node Start 5 D E G I Could use BFS on a map to plan driving routes between ciles Search adjacent ciles first Goal 4

5 BFS considers the number of steps, but not how long each step could take Fastest driving route to Sea3le from Hanover Sea_le 50 hours? 29 hours 12 hours Total <me: 45 hours Hanover 4 hours Could try to take the most direct route Take local roads Try to keep on a line between Start and Goal Could try to take major highways: New York Chicago Sea_le 5 Drive Lme eslmates from travelmath.com

6 Now we consider the idea that not all steps are the same Fastest driving route to Sea3le from Hanover 50 hours? 29 hours 12 hours Total <me: 45 hours 4 hours BFS would choose the direct route (one leg) Highway travel makes larger number of steps more a_raclve Note: our metric now is driving Lme, however total distance is longer! Drive Lme eslmates from travelmath.com Need a way to account for the idea that each step might have different weight (drive Lme here) 6

7 With no negalve edge weights, we can use Dijkstra s algorithm to find short paths Goal: find shortest path to all nodes considering edge weights Start at node s (single source) Find path with smallest sum of weights to all other nodes Store shortest path weights in v.dist instance variable Keep back pointer to previous node in v.pred Updated v.dist and v.pred if find shorter path later found 7

8 To get intuilon, imagine sending runners from the start to all adjacent nodes Time 0 s.dist = 0 Simula<on s.dist = 0 Runners take edge weight minutes to arrive at adjacent nodes When runners arrive at node: Record arrival Lme in v.dist Record prior node in v.pred Runners immediately leave for an adjacent node Here runners leave for y and t 8

9 Imagine we send runners from the start to all adjacent nodes Time 4 s.dist = 0 Runner arrives at y in 4 minutes Record y.dist = 4 Record y.pred = s Runners leave y for adjacent nodes t, x, and z y.dist = 4 y.pred = s Runner from s has not reached t yet 9

10 Imagine we send runners from the start to all adjacent nodes Time 5 t.dist = 5 t.pred = y s.dist = 0 Runner from y arrives at t at Lme 5 t.dist = 5 t.pred = y Runners from s slll hasn t made it to t y.dist = 4 y.pred = s Runners leave t for adjacent nodes x and y 10

11 Imagine we send runners from the start to all adjacent nodes Time 6 t.dist = 5 t.pred = y Runner from s arrives at t at Lme 6 s.dist = 0 Runner from y has already arrived, so best route is from y, not direct from s y.dist = 4 y.pred = s Do not update t.dist and t.pred NOTE: BFS would have chosen the direct route to t 11

12 Imagine we send runners from the start to all adjacent nodes Time 7 t.dist = 5 t.pred = y s.dist = 0 Runner from y arrives at z at Lme 7 Record z.dist = 7 and z.pred = y Runners leave z for s and x y.dist = 4 y.pred = s z.dist = 7 z.pred = y 12

13 Imagine we send runners from the start to all adjacent nodes Time 8 t.dist = 5 t.pred = y x.dist = 8 x.pred = t Runner from t arrives at x at Lme 8 x.dist = 8, x.pred = t s.dist = 0 All nodes explored Now have shortest path from s to all other nodes y.dist = 4 y.pred = s z.dist = 7 z.pred = y Shaded lines indicate best path to each node Path forms a tree on graph 13

14 Agenda 1. Shortest- path simulalon 2. Dijkstra s algorithm 3. A* search 4. Implicit graphs 14

15 Dijkstra s algorithms works similarly but doesn t rely on wailng for runners Dijkstra s algorithm Overview Start at s Process all out edges at the same Lme Compare distance to adjacent nodes with best so far If current path < best, update best distance and predecessor node Example: one hop from s set t.dist = 6, t.pred = s 15

16 Dijkstra s algorithms works similarly but doesn t rely on wailng for runners Dijkstra s algorithm Overview Start at s Process all out edges at the same Lme Compare distance to adjacent nodes with best so far If current path < best, update best distance and predecessor node Example: one hop from s set t.dist = 6, t.pred = s, then update t.dist = 5, t.pred = y 16 on second hop

17 Dijkstra uses a Min Priority Queue with dist values as keys to get closest vertex Dijkstra s algorithm star<ng from s void dijkstra(s) { queue = new PriorityQueue<Vertex>(); for (each vertex v) { v.dist = infinity; v.pred = null; queue.enqueue(v); s.dist = 0; IniLalize s distance Set up Min Priority Queue IniLalize dist and pred Use dist as key for Min Priority Queue (inilally infinite) while (!queue.isempty()) { u = queue.extractmin(); for (each vertex v adjacent to u) relax(u, v); While not all nodes have been explored Get closest node based on distance (inilally s) Examine adjacent and relax 17

18 Dijkstra defines a relax method to update best path if needed Dijkstra s relax method void relax(u, v) { if (u.dist + w(u,v) < v.dist) { v.dist = u.dist + w(u,v); v.pred = u; Currently at vertex u, considering distance to vertex v Check if distance to u + distance from u to v < best distance to v so far Distance from u to v is w(u,v) If shorter total distance to v than previous, then update: v.dist = u.dist + w(u,v) v.pred = u 18

19 Example Dijkstra s algorithm void dijkstra(s) { queue = new PriorityQueue<Vertex>(); for (each vertex v) { v.dist = infinity; v.pred = null; queue.enqueue(v); s.dist = 0; while (!queue.isempty()) { u = queue.extractmin(); for (each vertex v adjacent to u) relax(u, v); All nodes have distance Infinity, except Start with distance 0 Distances shown in center of verlces extractmin() from Min Priority Queue first selects s (dist =0) 19

20 Example Dijkstra s algorithm void dijkstra(s) { queue = new PriorityQueue<Vertex>(); for (each vertex v) { v.dist = infinity; v.pred = null; queue.enqueue(v); s.dist = 0; Loop over all adjacent nodes v If distance less than smallest so far, then relax That is the case here, so update dist and pred on t and y while (!queue.isempty()) { u = queue.extractmin(); for (each vertex v adjacent to u) relax(u, v); 20

21 Example Dijkstra s algorithm void dijkstra(s) { queue = new PriorityQueue<Vertex>(); for (each vertex v) { v.dist = infinity; v.pred = null; queue.enqueue(v); s.dist = 0; extractmin() now picks y (dist=4) Look at adjacent t,x, and z Relax each of them while (!queue.isempty()) { u = queue.extractmin(); for (each vertex v adjacent to u) relax(u, v); 21

22 Example Dijkstra s algorithm void dijkstra(s) { queue = new PriorityQueue<Vertex>(); for (each vertex v) { v.dist = infinity; v.pred = null; queue.enqueue(v); s.dist = 0; extractmin() now picks t (dist =5) Look at adjacent x and y Relax x, but not y while (!queue.isempty()) { u = queue.extractmin(); for (each vertex v adjacent to u) relax(u, v); 22

23 Example Dijkstra s algorithm void dijkstra(s) { queue = new PriorityQueue<Vertex>(); for (each vertex v) { v.dist = infinity; v.pred = null; queue.enqueue(v); s.dist = 0; extractmin() now picks z (dist = 7) Look at adjacent x and s Do not relax x or s while (!queue.isempty()) { u = queue.extractmin(); for (each vertex v adjacent to u) relax(u, v); 23

24 Example Dijkstra s algorithm s void dijkstra(s) { queue = new PriorityQueue<Vertex>(); for (each vertex v) { v.dist = infinity; v.pred = null; queue.enqueue(v); s.dist = 0; extractmin() now picks x (dist = 8) Look at adjacent z Do not relax z Done! while (!queue.isempty()) { u = queue.extractmin(); for (each vertex v adjacent to u) relax(u, v); 24

25 Run Lme complexity is O(n log n + m log n) Dijkstra s algorithm Add and remove each vertex once in Priority Queue Relax each edge (and perhaps reduce key) once O(n*(insert Lme + extractmin) + m*(reducekey)) If using heap- based Priority Queue, then each queue operalon takes O(log n) Total = O(n log n + m log n) Can implement with a Fibonacci heap with O(n 2 ) Take CS31 to find out how! 25

26 Agenda 1. Shortest- path simulalon 2. Dijkstra s algorithm 3. A* search 4. Implicit graphs 26

27 A* can help find the best path between two nodes faster than Dijkstra A* algorithm from Hanover to Boston Montpelier Randolph 25 Hanover Manchester EsLmate distance to goal (maybe use Euclidean distance) EsLmate must be actual distance (admissible) Distances non- negalve (distance monotone increasing) Boston EsLmated distance to goal Actual distance to node 27

28 A* can help find the best path between two nodes faster than Dijkstra A* algorithm from Hanover to Boston Montpelier Randolph 25 Hanover Manchester 55 Keep Priority Queue using distance so far + eslmate for each node ( open set ) Keep closed set where we know we already found the best route Boston EsLmated distance to goal Actual distance to node 28

29 A* can help find the best path between two nodes faster than Dijkstra Step 1: Start at Hanover, add to Open set Montpelier Randolph 25 Hanover Manchester Open set (Priority Queue) Hanover = 60 Closed set Boston EsLmated distance to goal Actual distance to node 29

30 A* can help find the best path between two nodes faster than Dijkstra Step 2: select min from Open set and explore adjacent Montpelier Randolph 25 Hanover Manchester Open set (Priority Queue) Randolph = 100 Manchester = = 110 Closed set Hanover = Boston EsLmated distance to goal Actual distance to node 30

31 A* can help find the best path between two nodes faster than Dijkstra Step 3: select min from Open set and explore adjacent Montpelier Randolph 25 Hanover Manchester 55 Open set (Priority Queue) Manchester = = 110 Montpelier = = 215 Closed set Hanover = 60 Randolph = 100 Boston EsLmated distance to goal Actual distance to node 31

32 A* can help find the best path between two nodes faster than Dijkstra Step 4: select min from Open set and explore adjacent Montpelier Randolph 25 Hanover Manchester 55 Open set (Priority Queue) Boston = = 120 Montpelier = = 215 Closed set Hanover = 60 Randolph = 100 Manchester = = 110 Boston EsLmated distance to goal Actual distance to node 32

33 A* can help find the best path between two nodes faster than Dijkstra Step 5: select min from Open set and explore adjacent Montpelier Randolph 25 Hanover Manchester 55 Open set (Priority Queue) Montpelier = = 215 Closed set Hanover = 60 Randolph = 100 Manchester = = 110 Boston = = 120 EsLmated distance to goal Actual distance to node Boston Found goal! No need to check Montpelier it can t be closer because a straight line would slll be greater than best path so far 33

34 Agenda 1. Shortest- path simulalon 2. Dijkstra s algorithm 3. A* search 4. Implicit graphs 34

35 Demo MazeSolver.java Run Load map 5 Try with: Stack == DFS Queue = BFS A* 35

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