[LintCode] Connecting Graph

Given n nodes in a graph labeled from 1 to n. There is no edges in the graph at beginning.

You need to support the following method:

1. connect(a, b), add an edge to connect node a and node b. 2.query(a, b)`, check if two nodes are connected

Example

5 // n = 5
query(1, 2) return false
connect(1, 2)
query(1, 3) return false
connect(2, 4)
query(1, 4) return true

Solution 1. BFS, O(1) connect, O(V) query, O(V + E) space

 1 public class ConnectingGraph {
 2     private ArrayList<HashSet<Integer>> lists = null;
 3     public ConnectingGraph(int n) {
 4         // initialize your data structure here.
 5         lists = new ArrayList<HashSet<Integer>>(n);
 6         for(int i = 0; i < n; i++){
 7             lists.add(new HashSet<Integer>());
 8         }
 9     }
10
11     public void connect(int a, int b) {
12         // Write your code here
13         lists.get(a - 1).add(b - 1);
14         lists.get(b - 1).add(a - 1);
15     }
16
17     public boolean query(int a, int b) {
18         // Write your code here
19         return bfs(a - 1, b - 1);
20     }
21
22     private boolean bfs(int source, int target){
23         Queue<Integer> queue = new LinkedList<Integer>();
24         HashSet<Integer> visited = new HashSet<Integer>();
25         queue.add(source);
26         visited.add(source);
27
28         while(!queue.isEmpty()){
29             int curr = queue.poll();
30             for(Integer i : lists.get(curr)){
31                 if(!visited.contains(i)){
32                     if(i == target){
33                         return true;
34                     }
35                     queue.add(i);
36                     visited.add(i);
37                 }
38             }
39         }
40         return false;
41     }
42 }

Solution 2. Union Find, O(1) connect and query on average, O(V) space(not storing edge information)

 1 class UnionFind {
 2     private int[] father = null;
 3     public UnionFind(int n){
 4         father = new int[n];
 5         for(int i = 0; i < n; i++){
 6             father[i] = i;
 7         }
 8     }
 9     public int find(int x){
10         if(father[x] == x){
11             return x;
12         }
13         return father[x] = find(father[x]);
14     }
15     public void connect(int a, int b){
16         int root_a = find(a);
17         int root_b = find(b);
18         if(root_a != root_b){
19             father[root_a] = root_b;
20         }
21     }
22 }
23 public class ConnectingGraph {
24     private UnionFind uf = null;
25     public ConnectingGraph(int n) {
26         // initialize your data structure here.
27         uf = new UnionFind(n);
28     }
29
30     public void connect(int a, int b) {
31         uf.connect(a - 1, b - 1);
32     }
33
34     public boolean query(int a, int b) {
35         return uf.find(a - 1) == uf.find(b - 1);
36     }
37 }

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