class Solution {
    public List<Integer> findMinHeightTrees(int n, int[][] edges) {
        List<Integer> res = new ArrayList<Integer>();
        //如果只有一個(gè)節(jié)點(diǎn)，則它就是最小高度樹
        if(n == 1){
            res.add(0);
            return res;
        }

        //每個(gè)節(jié)點(diǎn)的鄰居數(shù)量
        int [] degree = new int[n];
        //每個(gè)節(jié)點(diǎn)的鄰居
        HashMap<Integer,List<Integer>> map = new HashMap<>();

        for(int [] edge : edges){
            int a = edge[0];
            int b = edge[1];

            degree[a]++;
            degree[b]++;

            if(map.get(a) == null){
                map.put(a,new ArrayList<Integer>());//key:節(jié)點(diǎn)   value:鄰居
            }

            if(map.get(b) == null){
                map.put(b,new ArrayList<Integer>());//key:節(jié)點(diǎn)   value:鄰居
            }

            map.get(a).add(b);
            map.get(b).add(a);
        }

        //建立隊(duì)列
        LinkedList<Integer> leafNodes = new LinkedList<Integer>();//表示葉子節(jié)點(diǎn)
        //將所有度為1的節(jié)點(diǎn)入隊(duì)
        for(int i = 0;i < degree.length;i++){
            if(degree[i] == 1){
                leafNodes.add(i);
            }
        }

        while(leafNodes.size() > 0){
            res.clear();
            //每一層節(jié)點(diǎn)的數(shù)量
            int size = leafNodes.size();

            for(int i = 0;i < size;i++){
                int leaf = leafNodes.poll();
                //將當(dāng)前節(jié)點(diǎn)加入到結(jié)果集
                res.add(leaf);

                List<Integer> neighbors = map.get(leaf);
                //將出度減一，也就是將最外層的葉子節(jié)點(diǎn)剪掉
                for(int neighbor : neighbors){
                    degree[neighbor]--;
                    if(degree[neighbor] == 1){
                        //葉子節(jié)點(diǎn)入隊(duì)
                        leafNodes.add(neighbor);
                    }
                }
            }
        }
        return res;
    }
}