[LeetCode] 133. Clone Graph

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

 Thought process:
  1. Copy nodes:
    1. Use BFS to traverse the graph. Maintain the mappings from original nodes to clone nodes in a hashmap. Also use the hashmap to check if a node is visited.
  2. Copy nodes' neighbors.

Solution 1 (BFS):
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/**
 * 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 node;
        }
        
        Queue<UndirectedGraphNode> queue = new LinkedList<>();
        Map<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<>();
        queue.offer(node);
        map.put(node, new UndirectedGraphNode(node.label));
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                UndirectedGraphNode n = queue.poll();
                for (UndirectedGraphNode neighbor : n.neighbors) {
                    if (!map.containsKey(neighbor)) {
                        map.put(neighbor, new UndirectedGraphNode(neighbor.label));
                        queue.offer(neighbor);
                    }
                }
            }
        }
        
        for (UndirectedGraphNode n : map.keySet()) {
            for (UndirectedGraphNode neighbor : n.neighbors) {
                map.get(n).neighbors.add(map.get(neighbor));
            }
        }
        
        return map.get(node);
    }
}

Time complexity:
BFS takes O(V + E). Copying neighbors also takes O(V + E). The overall time complexity is O(V + E).

 Solution 2 (DFS):

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/**
 * 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) {
        Map<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<>();
        return cloneGraph(node, map);
    }
    
    private UndirectedGraphNode cloneGraph(UndirectedGraphNode node, Map<UndirectedGraphNode, UndirectedGraphNode> map) {
        if (node == null) {
            return null;
        }
        if (map.containsKey(node)) {
            return map.get(node);
        }
        
        UndirectedGraphNode clone = new UndirectedGraphNode(node.label);
        map.put(node, clone);
        
        for (UndirectedGraphNode neighbor : node.neighbors) {
            clone.neighbors.add(cloneGraph(neighbor, map));
        }
        return clone;
    }
}

Location: San Jose, CA, USA

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