src/main/java/com/fishercoder/solutions/_592.java

package com.fishercoder.solutions;

import java.util.ArrayList;
import java.util.List;

/**
 * 592. Fraction Addition and Subtraction
 *
 * Given a string representing an expression of fraction addition and subtraction,
 * you need to return the calculation result in string format.
 * The final result should be irreducible fraction.
 * If your final result is an integer,
 * say 2, you need to change it to the format of fraction that has denominator 1.
 * So in this case, 2 should be converted to 2/1.

 Example 1:
 Input:"-1/2+1/2"
 Output: "0/1"

 Example 2:
 Input:"-1/2+1/2+1/3"
 Output: "1/3"

 Example 3:
 Input:"1/3-1/2"
 Output: "-1/6"

 Example 4:
 Input:"5/3+1/3"
 Output: "2/1"

 Note:
 The input string only contains '0' to '9', '/', '+' and '-'. So does the output.
 Each fraction (input and output) has format ±numerator/denominator.
 If the first input fraction or the output is positive, then '+' will be omitted.
 The input only contains valid irreducible fractions,
 where the numerator and denominator of each fraction will always be in the range [1,10].
 If the denominator is 1, it means this fraction is actually an integer in a fraction format defined above.
 The number of given fractions will be in the range [1,10].
 The numerator and denominator of the final result are guaranteed to be valid and in the range of 32-bit int.
 */
public class _592 {

    /**Credit: https://discuss.leetcode.com/topic/89993/java-solution-fraction-addition-and-gcd*/
    public String fractionAddition(String expression) {
        List<String> nums = new ArrayList<>();
        int i = 0;
        int j = 0;
        while (j <= expression.length()) {
            if (j == expression.length() || j != i && (expression.charAt(j) == '-' || expression.charAt(j) == '+')) {
                if (expression.charAt(i) == '+') {
                    nums.add(expression.substring(i + 1, j));
                } else {
                    nums.add(expression.substring(i, j));
                }
                i = j;
            }
            j++;
        }

        String result = "0/1";
        for (String frac : nums) {
            result = add(result, frac);
        }
        return result;
    }

    private String add(String result, String frac) {
        String[] frac1 = frac.split("/");
        String[] frac2 = result.split("/");
        int n1 = Integer.parseInt(frac1[0]);
        int d1 = Integer.parseInt(frac1[1]);
        int n2 = Integer.parseInt(frac2[0]);
        int d2 = Integer.parseInt(frac2[1]);
        int numerator = n1 * d2 + n2 * d1;
        int denominator = d1 * d2;
        if (numerator == 0) {
            return "0/1";
        }

        boolean negative = numerator * denominator < 0;
        numerator = Math.abs(numerator);
        denominator = Math.abs(denominator);
        int gcd = getGCD(numerator, denominator);

        return (negative ? "-" : "") + (numerator / gcd) + "/" + (denominator / gcd);
    }

    private int getGCD(int a, int b) {
        if (a == 0 || b == 0) {
            return a + b;
        }
        return getGCD(b, a % b);
    }

}