Minimum Height Trees LeetCode solution in Java

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class Solution {
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  public List<Integer> findMinHeightTrees(int n, int[][] edges) {
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    if (edges.length == 0) {
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      List<Integer> al = new ArrayList<>();
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      al.add(0);
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      return al;
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    }
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    HashMap<Integer, Set<Integer>> map = new HashMap<>(); // map == graph
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    int[] degree = new int[n];
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    for (int[] edge : edges) {
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      int src = edge[0];
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      int dest = edge[1];
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      map.putIfAbsent(src, new HashSet<>());
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      map.get(src).add(dest);
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      map.putIfAbsent(dest, new HashSet<>());
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      map.get(dest).add(src);
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      degree[src]++;
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      degree[dest]++;
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    }
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    Queue<Integer> q = new ArrayDeque<>();
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    for (int i = 0; i < degree.length; i++) {
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      if (degree[i] == 1) {
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        q.offer(i);
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      }
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    }
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    int count = n;
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    while (count > 2) {
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      int size = q.size();
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      count -= size;
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      while (size-- > 0) {
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        Integer src = q.poll();
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        for (Integer connection : map.get(src)) {
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          degree[connection]--;
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          if (degree[connection] == 1) {
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            q.offer(connection);
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          }
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        }
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      }
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    }
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    return new ArrayList<>(q);
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  }
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}