Add Binary LeetCode solution in Java

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class Solution {
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  public String addBinary(String a, String b) {
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    StringBuilder res = new StringBuilder();
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    int i = a.length() - 1;
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    int j = b.length() - 1;
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    int carry = 0;
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    while (i >= 0 || j >= 0) {
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      int sum = carry;
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      if (i >= 0) sum += a.charAt(i--) - '0';
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      if (j >= 0) sum += b.charAt(j--) - '0';
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      carry = sum > 1 ? 1 : 0;
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      res.append(sum % 2);
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    }
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    if (carry != 0) res.append(carry);
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    return res.reverse().toString();
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  }
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}