Graph Valid Tree

Java

public class Solution {
    /**
     * @param n: An integer
     * @param edges: a list of undirected edges
     * @return: true if it's a valid tree, or false
     */
		public boolean validTree(int n, int[][] edges) {
			if (n == 0) return false;
			if (edges.length != n - 1) return false;//edge + 1 = n  Valid Tree 的必要条件_套路
			
			Map<Integer, Set<Integer>> graph = initGraph(n, edges);
			
			Queue<Integer> queue = new LinkedList<Integer>();
			Set<Integer> hash = new HashSet<>();
			
			queue.offer(0);
			hash.add(0);
			while (!queue.isEmpty()) {
				int cur = queue.poll();
				for (int nei : graph.get(cur)) {
					if (!hash.contains(nei)) {
						queue.offer(nei);
		hash.add(nei);                         //BFS套路
					}
				}
			}
			return hash.size() == n;
		}
		
		//HashMap<key-node, value-connected nodes>  
		
		private Map<Integer, Set<Integer>> initGraph(int n, int[][] edges) {
			Map<Integer, Set<Integer>> graph = new HashMap<Integer, Set<Integer>>();
			for (int i = 0; i < n; i++) {
				graph.put(i, new HashSet<Integer>());  //用节点关系建立graph
			}
			for (int j = 0; j < edges.length; j++) {
				int u = edges[j][0];
				int v = edges[j][1];
				graph.get(u).add(v);
				graph.get(v).add(u); // graph.add(graph.get(v), u);这里为什么不对??
			}
			return graph;
		}
}