Divide a String Into Groups of Size k LeetCode solution in Java

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class Solution {
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  public String[] divideString(String s, int k, char fill) {
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    // the size of result array
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    String[] r = new String[(int) Math.ceil((s.length() * 1.0) / k)];
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    int idx;
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    // if the last one needs to be filled, then I do it
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    if (s.length() % k != 0) {
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      idx = s.length() - (s.length() % k);
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      StringBuilder sb = new StringBuilder();
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      sb.append(s.substring(idx));
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      for (int i = sb.length(); i < k; i++) {
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        sb.append(fill);
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      }
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      r[r.length - 1] = sb.toString();
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    } else {
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      // no need to fill the last,
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      // that's why idx is equal to s length
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      idx = s.length();
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    }
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    for (int i = 0; i < idx; i += k) {
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      r[i / k] = s.substring(i, i + k);
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    }
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    return r;
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  }
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}