Replace All ?'s to Avoid Consecutive Repeating Characters LeetCode solution in Java

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class Solution {
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  public String modifyString(String s) {
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    if (s.length() == 1) {
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      return String.valueOf('a');
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    }
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    if (s.length() == 0) {
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      return null;
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    }
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    StringBuilder stringBuilder = new StringBuilder(s);
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    for (int i = 0; i < stringBuilder.length(); i++) {
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      if (stringBuilder.charAt(i) == '?') {
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        for (char j = 'a'; j <= 'z'; j++) {
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          if (i != 0
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              && i != stringBuilder.length() - 1
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              && j != stringBuilder.charAt(i - 1)
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              && j != stringBuilder.charAt(i + 1)) {
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            stringBuilder.replace(i, i + 1, String.valueOf(j));
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          }
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          if (i == 0 && j != stringBuilder.charAt(i + 1)) {
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            stringBuilder.replace(i, i + 1, String.valueOf(j));
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          }
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          if (i == stringBuilder.length() - 1 && j != stringBuilder.charAt(i - 1)) {
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            stringBuilder.replace(i, i + 1, String.valueOf(j));
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          }
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        }
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      }
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    }
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    return stringBuilder.toString();
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  }
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}