Avoid Flood in The City LeetCode solution in Java

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class Solution {
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  public int[] avoidFlood(int[] rains) {
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    // the previous appeared idx of rains[i]
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    Map<Integer, Integer> map = new HashMap<>();
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    TreeSet<Integer> zeros = new TreeSet<>();
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    int[] res = new int[rains.length];
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    for (int i = 0; i < rains.length; i++) {
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      if (rains[i] == 0) {
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        zeros.add(i);
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      } else {
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        if (map.containsKey(rains[i])) {
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          // find the location of zero that can be used to empty rains[i]
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          Integer next = zeros.ceiling(map.get(rains[i]));
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          if (next == null) return new int[0];
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          res[next] = rains[i];
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          zeros.remove(next);
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        }
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        res[i] = -1;
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        map.put(rains[i], i);
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      }
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    }
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    for (int i : zeros) res[i] = 1;
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    return res;
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  }
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}