最小高度的树 Minimum Height Trees

    public List<Integer> findMinHeightTrees(int n, int[][] edges) {
        // tricks ：使用Collections.singletonList(0)可以直接返回单个值的List
        if (n == 1) return Collections.singletonList(0);
        // tricks ：使用Set是为了方便在之后维护graph的时候方便按值删除
        List<Set<Integer>> graph = new ArrayList<>(n);
        for (int i = 0; i < n; i++) graph.add(new HashSet<>());
        for (int[] edge : edges) {
            graph.get(edge[0]).add(edge[1]);
            graph.get(edge[1]).add(edge[0]);
        }
        List<Integer> leaves = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            if (graph.get(i).size() == 1) leaves.add(i);
        }
        while (n > 2) {
            n -= leaves.size();
            List<Integer> newLeaves = new ArrayList<>();
            for (int i : leaves) {
                int j = graph.get(i).iterator().next();
                graph.get(j).remove(i);
                if (graph.get(j).size() == 1) newLeaves.add(j);
            }
            leaves = newLeaves;
        }
        return leaves;
    }