Decrypt String from Alphabet to Integer Mapping LeetCode solution in Java

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class Solution {
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  public String freqAlphabets(String str) {
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    HashMap<String, Character> map = new HashMap<>();
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    int k = 1;
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    for (char ch = 'a'; ch <= 'z'; ch++) {
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      if (ch < 'j') map.put(String.valueOf(k++), ch);
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      else map.put(String.valueOf(k++) + "#", ch);
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    }
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    StringBuilder sb = new StringBuilder();
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    int i = str.length() - 1;
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    while (i >= 0) {
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      if (str.charAt(i) == '#') {
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        sb.append(map.get(str.substring(i - 2, i + 1)));
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        i -= 3;
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      } else {
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        sb.append(map.get(str.substring(i, i + 1)));
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        i--;
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      }
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    }
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    return sb.reverse().toString();
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  }
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}