Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 4 | Trees and Graphs
4.2 Given a directed graph, design an algorithm to find out whether there is a route between
two nodes.
SOLUTION
pg 54
This problem can be solved by just simple graph traversal, such as depth first search or
breadth first search. We start with one of the two nodes and, during traversal, check if the
other node is found. We should mark any node found in the course of the algorithm as ‘already
visited’ to avoid cycles and repetition of the nodes.
1 public enum State {
2 Unvisited, Visited, Visiting;
3 }
4
5 public static boolean search(Graph g, Node start, Node end) {
6 LinkedList q = new LinkedList(); // operates as Stack
7 for (Node u : g.getNodes()) {
8 u.state = State.Unvisited;
9 }
10 start.state = State.Visiting;
11 q.add(start);
12 Node u;
13 while(!q.isEmpty()) {
14 u = q.removeFirst(); // i.e., pop()
15 if (u != null) {
16 for (Node v : u.getAdjacent()) {
17 if (v.state == State.Unvisited) {
18 if (v == end) {
19 return true;
20 } else {
21 v.state = State.Visiting;
22 q.add(v);
23 }
24 }
25 }
26 u.state = State.Visited;
27 }
28 }
29 return false;
30 }
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