Calculate Digit Sum of a String LeetCode solution in Java

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class Solution {
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  public String digitSum(String s, int k) {
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    while (s.length() > k) s = gen(s, k);
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    return s;
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  }
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  public String gen(String s, int k) {
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    String res = "";
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    for (int i = 0; i < s.length(); ) {
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      int count = 0, num = 0;
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      while (i < s.length() && count++ < k) num += Character.getNumericValue(s.charAt(i++));
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      res += num;
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    }
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    return res;
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  }
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}