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[LintCode] Six Degrees

Six degrees of separation is the theory that everyone and everything is six or fewer steps away, by way of introduction, from any other person in the world, so that a chain of "a friend of a friend" statements can be made to connect any two people in a maximum of six steps.

Given a friendship relations, find the degrees of two people, return -1 if they can not been connected by friends of friends.

Example

Gien a graph:

1------2-----4
           /
          /
   --3--/

{1,2,3#2,1,4#3,1,4#4,2,3} and s = 1, t = 4 return 2

Gien a graph:

1      2-----4
             /
           /
          3

{1#2,4#3,4#4,2,3} and s = 1, t = 4 return -1

 1 /**
 2  * Definition for Undirected graph.
 3  * class UndirectedGraphNode {
 4  *     int label;
 5  *     List<UndirectedGraphNode> neighbors;
 6  *     UndirectedGraphNode(int x) { 
 7  *         label = x;
 8  *         neighbors = new ArrayList<UndirectedGraphNode>(); 
 9  *     }
10  * };
11  */
12 
13 
14 public class Solution {
15     public int sixDegrees(List<UndirectedGraphNode> graph, UndirectedGraphNode s, UndirectedGraphNode t) {
16         if(graph == null || !graph.contains(s) || !graph.contains(t)) {
17             return -1;
18         }
19         if(s == t) {
20             return 0;
21         }
22         Queue<UndirectedGraphNode> q = new LinkedList<UndirectedGraphNode>();
23         Set<UndirectedGraphNode> visited = new HashSet<UndirectedGraphNode>();
24         int layer = 1;
25         q.add(s);
26         visited.add(s);
27         while(!q.isEmpty()) {
28             int size = q.size();
29             for(int i = 0; i < size; i++) {
30                  UndirectedGraphNode curr = q.poll();
31                  for(UndirectedGraphNode neighbor : curr.neighbors) {
32                      if(visited.contains(neighbor)) {
33                          continue;
34                      }
35                      if(neighbor == t) {
36                         return layer; 
37                      }
38                      else {
39                          q.add(neighbor);
40                          visited.add(neighbor);
41                      }
42                  }
43             }
44             layer++;
45         }
46         return -1;
47     }
48 }