Minimum Height Trees

For an undirected graph with tree characteristics, we can choose any node as the root. The result graph is then a rooted tree. Among all possible rooted trees, those with minimum height are called minimum height trees (MHTs). Given such a graph, write a function to find all the MHTs and return a list of their root labels.

Format
The graph contains n nodes which are labeled from 0 to n - 1. You will be given the number n and a list of undirected edges (each edge is a pair of labels).

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.

Example 1 :

Input: n = 4, edges = [[1, 0], [1, 2], [1, 3]]

        0
        |
        1
       / \
      2   3 

Output: [1]

Example 2 :

Input: n = 6, edges = [[0, 3], [1, 3], [2, 3], [4, 3], [5, 4]]

     0  1  2
      \ | /
        3
        |
        4
        |
        5 

Output: [3, 4]

思路：從外圍一層一層的往裏面減；類似於topological sort，每次indegree是1的時候，就加入；因爲是無向圖，所以indegree == 1的時候就是最外面的leaf點；收集每一層，返回最後一層的list就可以了；這個思路太屌了；

class Solution {
    public List<Integer> findMinHeightTrees(int n, int[][] edges) {
        List<Integer> list = new ArrayList<Integer>();
        if(n == 1) {
            list.add(0);
            return list;
        }
        
        int[] indegree = new int[n];
        HashMap<Integer, HashSet<Integer>> hashmap = new HashMap<Integer, HashSet<Integer>>();
        for(int i = 0; i < n; i++) {
            hashmap.put(i, new HashSet<Integer>());
        }
        
        for(int i = 0; i < edges.length; i++) {
            int a = edges[i][0];
            int b = edges[i][1];
            
            hashmap.get(a).add(b);
            hashmap.get(b).add(a);
            indegree[a]++;
            indegree[b]++;
        }
        
        Queue<Integer> queue = new LinkedList<Integer>();
        for(int i = 0; i < indegree.length; i++) {
            if(indegree[i] == 1) {
                queue.offer(i);
            }
        }
        
        while(!queue.isEmpty()) {
            list = new ArrayList<Integer>();
            int size = queue.size();
            for(int i = 0; i < size; i++) {
                Integer head = queue.poll();
                list.add(head);
                for(Integer neighbor: hashmap.get(head)) {
                    indegree[neighbor]--;
                    if(indegree[neighbor] == 1){
                        queue.offer(neighbor);
                    }
                }
            }
        }
        return list;
    }
}