Poison

133. Clone Graph

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class Solution {
    public Node cloneGraph(Node node) {
        if (node == null) {
            return null;
        }

        Map<Node, Node> originalToCloneNodeMap = new IdentityHashMap<>();
        return cloneNode(node, originalToCloneNodeMap);
    }

    private List<Node> cloneNeighbors(List<Node> neighbors, Map<Node, Node> originalToCloneNodeMap) {
        if (neighbors.isEmpty()) {
            return Collections.emptyList();
        }

        List<Node> cloneNeighbors = new ArrayList<>(neighbors.size());
        for (Node node : neighbors) {
            cloneNeighbors.add(cloneNode(node, originalToCloneNodeMap));
        }

        return cloneNeighbors;
    }

    private Node cloneNode(Node node, Map<Node, Node> originalToCloneNodeMap) {
        Node cloneNode = originalToCloneNodeMap.get(node);
        if (cloneNode != null) {
            return cloneNode;
        }

        cloneNode = new Node(node.val);
        originalToCloneNodeMap.put(node, cloneNode);
        cloneNode.neighbors = cloneNeighbors(node.neighbors, originalToCloneNodeMap);
        return cloneNode;
    }
}
Reference

133. Clone Graph