Apply Discount to Prices LeetCode solution in Java

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class Solution {
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  public String discountPrices(String sentence, int discount) {
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    String x[] = sentence.split(" ");
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    StringBuilder sb = new StringBuilder();
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    for (String s : x) {
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      if (isPrice(s)) sb.append(calc(Double.parseDouble(s.substring(1)), discount) + " ");
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      else sb.append(s + " ");
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    }
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    sb.deleteCharAt(sb.length() - 1);
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    return sb.toString();
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  }
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  boolean isPrice(String s) {
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    return s.startsWith("$") && s.substring(1).matches("\\d+");
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  }
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  String calc(double num, double discount) {
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    double ans = num - (double) ((double) num * discount / 100.00);
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    return "$" + String.format("%.2f", ans);
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  }
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}