133. Clone Graph

Given a reference of a node in a connected undirected graph, return a deep copy (clone) of the graph. Each node in the graph contains a val (int) and a list (List[Node]) of its neighbors.

Example:

Input:
{"$id":"1","neighbors":[{"$id":"2","neighbors":[{"$ref":"1"},{"$id":"3","neighbors":[{"$ref":"2"},{"$id":"4","neighbors":[{"$ref":"3"},{"$ref":"1"}],"val":4}],"val":3}],"val":2},{"$ref":"4"}],"val":1}

Explanation:
Node 1's value is 1, and it has two neighbors: Node 2 and 4.
Node 2's value is 2, and it has two neighbors: Node 1 and 3.
Node 3's value is 3, and it has two neighbors: Node 2 and 4.
Node 4's value is 4, and it has two neighbors: Node 1 and 3.

Note:

1. The number of nodes will be between 1 and 100.

2. The undirected graph is a simple graph, which means no repeated edges and no self-loops in the graph.

3. Since the graph is undirected, if node p has node q as neighbor, then node q must have node p as neighbor too.

4. You must return the copy of the given node as a reference to the cloned graph.

// Recursive DFS
Node* helper(Node* node, unordered_map<Node*, Node*>& m) {
    if (!node) return nullptr;
    if (m.count(node)) return m[node];
    Node* newNode = new Node(node->val, vector<Node*>());
    m[node] = newNode;
    for (Node* &neighbor : node->neighbors) {
        newNode->neighbors.push_back(helper(neighbor, m));
    }
    return newNode;
}
Node* cloneGraph(Node* node) { // time: O(n); space: O(n)
    unordered_map<Node*, Node*> m;
    return helper(node, m);
}

// Iterative BFS
Node* cloneGraph(Node* node) { // time: O(n); space: O(n)
    if (!node) return nullptr;
    unordered_map<Node*, Node*> m;
    Node* copy = new Node(node->val, vector<Node*>());
    m[node] = copy;
    queue<Node*> q({node});
    while (!q.empty()) {
        Node* cur = q.front(); q.pop();
        for (Node* &neigh : cur->neighbors) {
            if (!m.count(neigh)) {
                Node* newNode = new Node(neigh->val, vector<Node*>());
                m[neigh] = newNode;
                q.push(neigh);
            }
            m[cur]->neighbors.push_back(m[neigh]);
        }
    }
    return copy;
}