Minimum Genetic Mutation LeetCode solution in Java

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class Solution {
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  public int minMutation(String start, String end, String[] bank) {
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    Set<String> set = new HashSet<>();
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    for (String tmp : bank) {
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      set.add(tmp);
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    }
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    if (!set.contains(end)) return -1;
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    if (start.equals(end)) return 0;
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    char[] var = {'A', 'C', 'G', 'T'};
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    Queue<String> q = new LinkedList<>();
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    q.add(start);
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    int count = 0;
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    while (!q.isEmpty()) {
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      int size = q.size();
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      for (int i = 0; i < size; i++) {
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        String str = q.poll();
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        char[] tmp = str.toCharArray();
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        if (str.equals(end)) return count;
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        for (int j = 0; j < 8; j++) {
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          char ch = tmp[j];
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          for (int k = 0; k < 4; k++) {
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            tmp[j] = var[k];
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            String node = new String(tmp);
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            if (set.contains(node)) {
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              q.add(node);
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              set.remove(node);
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            }
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          }
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          tmp[j] = ch;
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        }
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      }
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      count++;
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    }
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    return -1;
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  }
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}