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class Solution {
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  public String reverseStr(String s, int k) {
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    char[] ch = s.toCharArray();
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    int cnt = 1, i = 0;
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    StringBuilder sb = new StringBuilder();
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    String ans = "";
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    if (k >= s.length()) {
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      sb.append(s);
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      sb.reverse();
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      return sb.toString();
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    }
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    for (i = 0; i < s.length() - k; i += k) {
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      String str = s.substring(i, i + k);
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      if (cnt % 2 != 0) {
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        sb.append(str);
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        sb.reverse();
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        ans += sb.toString();
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        cnt++;
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        sb = new StringBuilder();
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      } else {
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        ans += str;
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        cnt++;
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      }
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    }
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    if (cnt % 2 != 0) {
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      sb.append(s.substring(i, s.length()));
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      sb.reverse();
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      ans += sb.toString();
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    } else {
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      ans += s.substring(i, s.length());
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    }
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    return ans;
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  }
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}