Base 7 LeetCode solution in Java

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class Solution {
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  public String convertToBase7(int num) {
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    StringBuilder sb = new StringBuilder();
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    while (Math.abs(num) > 6) {
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      sb.append(num % 7);
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      num = num / 7;
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    }
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    if (num < 0) {
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      sb.append(Math.abs(num)).append("-");
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    } else {
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      sb.append(num);
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    }
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    for (int i = 0; i < sb.length() - 1; i++) {
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      if (!Character.isDigit(sb.charAt(i))) {
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        sb.replace(i, i + 1, "");
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      }
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    }
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    return sb.reverse().toString();
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  }
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}