Fraction Addition and Subtraction LeetCode solution in Java

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class Solution {
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  // Great Common Divisor
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  private int gcd(int a, int b) {
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    int r = b % a;
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    if (r == 0) return a;
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    return gcd(r, a);
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  }
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  private int[] add(int[] a, int[] b) {
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    if (a[0] == 0) return b;
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    if (b[0] == 0) return a;
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    int[] ret = new int[] {a[0] * b[1] + b[0] * a[1], a[1] * b[1]};
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    if (ret[0] == 0) {
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      ret[1] = 1;
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      return ret;
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    }
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    int sign = ret[0] < 0 ? -1 : 1;
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    ret[0] *= sign;
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    int g = gcd(Math.min(ret[0], ret[1]), Math.max(ret[0], ret[1]));
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    if (g > 1) {
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      ret[0] /= g;
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      ret[1] /= g;
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    }
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    ret[0] *= sign;
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    return ret;
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  }
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  public String fractionAddition(String expression) {
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    return Stream.of(expression.replaceAll("\\-", "+-").split("\\+"))
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        .filter(s -> s.length() > 0)
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        .map(s -> s.split("/"))
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        .map(a -> new int[] {Integer.parseInt(a[0]), Integer.parseInt(a[1])})
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        .reduce((a, b) -> add(a, b))
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        .map(p -> p[0] + "/" + p[1])
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        .get();
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  }
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}