Masking Personal Information LeetCode solution in Java

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class Solution {
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  public String maskPII(String s) {
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    StringBuilder sb = new StringBuilder();
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    // email handeling
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    if ((s.charAt(0) >= 97 && s.charAt(0) <= 122) || (s.charAt(0) >= 65 && s.charAt(0) <= 90)) {
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      s = s.toLowerCase();
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      int indexofAt = s.indexOf('@');
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      String firstName = s.substring(0, indexofAt);
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      sb.append(firstName.charAt(0))
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          .append("*****")
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          .append(firstName.charAt(firstName.length() - 1));
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      sb.append(s.substring(indexofAt, s.length()));
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    }
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    // phone number handeling
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    else {
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      int digits = 0;
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      for (int i = 0; i < s.length(); i++) {
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        if (Character.isDigit(s.charAt(i))) {
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          digits++;
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        }
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      }
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      if (digits > 10) {
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        sb.append('+');
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      }
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      while (digits > 10) {
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        sb.append('*');
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        digits--;
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      }
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      if (sb.toString().isEmpty() == false) {
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        sb.append('-');
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      }
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      sb.append("***").append('-').append("***-");
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      StringBuilder last4 = new StringBuilder();
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      int count = 0;
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      for (int i = s.length() - 1; i >= 0; --i) {
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        if (count == 4) {
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          break;
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        }
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        if (Character.isDigit(s.charAt(i))) {
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          last4.append(s.charAt(i));
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          count++;
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        }
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      }
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      sb.append(last4.reverse());
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    }
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    return sb.toString();
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  }
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}