Graph Valid Tree.java

public boolean validTree(int n, int[][] edges) {
    HashMap<Integer, ArrayList<Integer>> map = new HashMap<Integer, ArrayList<Integer>>();
    for(int i=0; i<n; i++){
        ArrayList<Integer> list = new ArrayList<Integer>();
        map.put(i, list);
    }
 
    for(int[] edge: edges){
        map.get(edge[0]).add(edge[1]);
        map.get(edge[1]).add(edge[0]);
    }
 
    boolean[] visited = new boolean[n];
 
    LinkedList<Integer> queue = new LinkedList<Integer>();
    queue.offer(0);
    while(!queue.isEmpty()){
        int top = queue.poll();
        if(visited[top])
            return false;
 
        visited[top]=true;
 
        for(int i: map.get(top)){
            if(!visited[i])
                queue.offer(i);
        }
    }
 
    for(boolean b: visited){
        if(!b)
            return false;
    }
 
    return true;
}