133. Clone Graph

Clone an undirected graph. Each node in the graph contains a label and a list of its neighbors.

OJ's undirected graph serialization:

Nodes are labeled uniquely.

We use # as a separator for each node, and , as a separator for node label and each neighbor of the node.

As an example, consider the serialized graph {0,1,2#1,2#2,2}.

The graph has a total of three nodes, and therefore contains three parts as separated by #.

1. First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
2. Second node is labeled as 1. Connect node 1 to node 2.
3. Third node is labeled as 2. Connect node 2 to node 2 (itself), thus forming a self-cycle.

Visually, the graph looks like the following:

       1
      / \
     /   \
    0 --- 2
         / \
         \_/

Solution:

Method 1: BFS

We use BFS to traverse the graph.

When we meet a node, we make a copy of it and store it into the HashMap.

We also check all its neighbors.

If any of its neighbor is not in the HashMap, we make a copy of it and store in the HashMap.

One more step: add this neighbor to the neighbor list of the current node we check.

The time complexity is O(n) and space complexity is also O(n).



Code:

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) {
            return null;
        }
        
        HashMap<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<>();
        Queue<UndirectedGraphNode> queue = new LinkedList<>();
        
        queue.offer(node);
        map.put(node, new UndirectedGraphNode(node.label));
        
        while (!queue.isEmpty()) {
            UndirectedGraphNode curr = queue.poll();
            for (UndirectedGraphNode nei : curr.neighbors) {
                if (!map.containsKey(nei)) {
                    map.put(nei, new UndirectedGraphNode(nei.label));
                    queue.add(nei);
                }
                map.get(curr).neighbors.add(map.get(nei));
            }
        }
        return map.get(node);
    }
}