1. 有向图 找所有start node到end node之间的路径输入是一个txt 形式如下：A EA : B C D. B : CC : ED : B.
输出一个List<String> 是从A到E所有的path

 1 package pocketGems;
 2
 3 import java.io.*;
 4 import java.util.ArrayList;
 5 import java.util.List;
 6 import java.util.HashMap;
 7 import java.util.HashSet;
 8 import java.util.Set;
 9
10 public class PathFinder2 {
11     public static void main(String[] args)
12             throws FileNotFoundException, IOException {
13         String filename = "C:/Users/yang liu/workspace/Interview2017/src/pocketGems/input_1.txt";
14         if (args.length > 0) {
15             filename = args[0];
16         }
17
18
19         List<String> answer = parseFile(filename);
20         System.out.println(answer);
21     }
22
23     static List<String> parseFile(String filename)
24             throws FileNotFoundException, IOException {
25         /*
26          *  Don't modify this function
27          */
28         BufferedReader input = new BufferedReader(new FileReader(filename));
29         List<String> allLines = new ArrayList<String>();
30         String line;
31         while ((line = input.readLine()) != null) {
32             allLines.add(line);
33         }
34         input.close();
35
36
37         return parseLines(allLines);
38     }
39
40     static List<String> parseLines(List<String> allLines) {
41         HashMap<String, HashSet<String>> graph = new HashMap<String, HashSet<String>>();
42         Set<String> visited = new HashSet<String>();
43         String src = allLines.get(0).split(" ")[0];
44         String dst = allLines.get(0).split(" ")[1];
45         for (int i=1; i<allLines.size(); i++) {
46             String line = allLines.get(i);
47             String node = line.trim().split(":")[0].trim();
48             String[] neibors = line.split(":")[1].trim().split(" ");
49             graph.put(node, new HashSet<String>());
50             for (String nb : neibors) {
51                 if (nb.length() != 0) graph.get(node).add(nb);
52             }
53         }
54
55         List<String> res = new ArrayList<String>();
56         visited.add(src);
57         helper(res, src, src, dst, visited, graph);
58         return res;
59     }
60
61     static void helper(List<String> res, String path, String cur, String dst, Set<String> visited, HashMap<String, HashSet<String>> graph) {
62         if (cur.equals(dst)) {
63             res.add(path);
64             return;
65         }
66         HashSet<String> neibors = graph.get(cur);
67         if (neibors != null) {
68             for (String each : neibors) {
69                 if (visited.contains(each)) continue;
70                 visited.add(each);
71                 helper(res, path+each, each, dst, visited, graph);
72                 visited.remove(each);
73             }
74         }
75     }
76 }