https://leetcode.com/problems/minimum-height-trees/

minimum-height-trees.java

class Solution {
    public List<Integer> findMinHeightTrees(int n, int[][] edges) {
        Set<Integer>[] graph = new Set[n];
        for (int i = 0; i < n; i++) {
            graph[i] = new HashSet<>();
        }
        
        for (int[] edge : edges) {
            graph[edge[0]].add(edge[1]);
            graph[edge[1]].add(edge[0]);
        }
        
        List<Integer> queue = new LinkedList<>();
        
        for (int i = 0; i < n; i++) {
            if (graph[i].size() <= 1) {
                queue.add(i);
            }
        }
        
        while(n > 2) {
            List<Integer> queue2 = new LinkedList<>();
            for (int i : queue) {
                n -= 1;
                for(int next : graph[i]) {
                    graph[next].remove(i);
                    if (graph[next].size() == 1) {
                        queue2.add(next);
                    }
                }
            }
            queue.clear();
            queue = queue2;
        }
        
        return queue;
    }
}