标签：
Given n nodes labeled from 0 to n - 1 and a list of undirected edges (each edge is a pair of nodes), write a function to check whether these edges make up a valid tree.
For example:
Given n = 5 and edges = [[0, 1], [0, 2], [0, 3], [1, 4]], return true.
Given n = 5 and edges = [[0, 1], [1, 2], [2, 3], [1, 3], [1, 4]], return false.
Hint:
Given n = 5 and edges = [[0, 1], [1, 2], [3, 4]], what should your return? Is this case a valid tree? Show More Hint Note: you can assume that no duplicate edges will appear in edges. Since all edges are undirected, [0, 1] is the same as [1, 0] and thus will not appear together in edges.
1 public boolean validTree(int n, int[][] edges) { 2 HashMap<Integer, ArrayList<Integer>> map = new HashMap<Integer, ArrayList<Integer>>(); 3 for(int i=0; i<n; i++){ 4 ArrayList<Integer> list = new ArrayList<Integer>(); 5 map.put(i, list); 6 } 7 8 for(int[] edge: edges){ 9 map.get(edge[0]).add(edge[1]); 10 map.get(edge[1]).add(edge[0]); 11 } 12 13 boolean[] visited = new boolean[n]; 14 15 LinkedList<Integer> queue = new LinkedList<Integer>(); 16 queue.offer(0); 17 while(!queue.isEmpty()){ 18 int top = queue.poll(); 19 if(visited[top]) 20 return false; 21 22 visited[top]=true; 23 24 for(int i: map.get(top)){ 25 if(!visited[i]) 26 queue.offer(i); 27 } 28 } 29 30 for(boolean b: visited){ 31 if(!b) 32 return false; 33 } 34 35 return true; 36 }
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原文地址：http://www.cnblogs.com/beiyeqingteng/p/5958915.html
