This is a leetcode problem 133 Clone Graph

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

• use BFS to solve this problem.
• use a Map in javascript to store a mapping between nodes in original graph and nodes in new graph.
• when using BFS to traverse the original graph, if a neighbor is already in the map, it means the corresponding node has been created and we just need to connect two nodes in the new graph
• when using BFS to traverse the original graph, if a neighbor isn’t in the map, we create a new node in the new graph, push the neighbor in the queue and connect two nodes in the new graph.

/**
 * @param {UndirectedGraphNode} graph
 * @return {UndirectedGraphNode}
 */
var cloneGraph = function(graph) {
    if (graph === null) {
        return null;
    }
    //bfs
    var queue = [];
    queue.push(graph);
    var map = new Map();
    var head = new UndirectedGraphNode(graph.label);
    map.set(graph, head);
    
    while (queue.length !== 0) {
        var curr = queue.shift();
        // console.log(curr);
        var neighbors = curr.neighbors;
        for (var i = 0; i < neighbors.length; i++) {
            var neighbor = neighbors[i];
            // this neighbor has not been visited
            if (map.get(neighbor) === undefined) {
                var copyNeighbor = new UndirectedGraphNode(neighbor.label);
                map.set(neighbor, copyNeighbor);
                map.get(curr).neighbors.push(copyNeighbor);
                queue.push(neighbor);
            // this neighbor has been visited
            } else {
               map.get(curr).neighbors.push(map.get(neighbor));
            }
        }
    }
    
    return head;
};