1using System;
2
3public class Solution {
4    public string FractionAddition(string expression) {
5        int numerator = 0, denominator = 1, num, dem;
6        int sign = 1;
7        for (int i = 0; i < expression.Length; ) {
8            if (expression[i] == '-' || expression[i] == '+') {
9                sign = expression[i] == '-' ? -1 : 1;
10                i++;
11            }
12            int start = i;
13            while (expression[i] != '/') i++;
14            num = int.Parse(expression.Substring(start, i - start)) * sign;
15            i++;
16            start = i;
17            while (i < expression.Length && Char.IsDigit(expression[i])) i++;
18            dem = int.Parse(expression.Substring(start, i - start));
19
20            int lcmDen = denominator * (dem / GCD(denominator, dem));
21            numerator = numerator * (lcmDen / denominator) + num * (lcmDen / dem);
22            denominator = lcmDen;
23        }
24
25        int gcd = GCD(Math.Abs(numerator), denominator);
26        numerator /= gcd;
27        denominator /= gcd;
28        return numerator + "/" + denominator;
29    }
30
31    private int GCD(int a, int b) {
32        return b == 0 ? a : GCD(b, a % b);
33    }
34}

1
public class Solution {
    public string FractionAdditionConsec(string expression) {
        int num = 0, den = 1, n, d;
        int i = 0, sign = 1;

        while (i < expression.Length) {
            if (expression[i] == '+' || expression[i] == '-') {
                sign = (expression[i] == '-') ? -1 : 1;
                i++;
            }

            int numBegin = i;
            while (expression[i] != '/') i++;
            n = int.Parse(expression.Substring(numBegin, i - numBegin));
            n *= sign;
            i++;

            int dBegin = i;
            while (i < expression.Length && Expression[i] >= '0' && expression[i] <= '9') i++;
            d = int.Parse(expression.Substring(dBegin, i - dBegin));

            int lcmDenom = den * (d / GCD(den, d));
            num = num * (lcmDenom / den) + n * (lcmDenom / d);
            den = lcmDenom;
        }

        int gcd = GCD(Math.Abs(num), den);
        return (num / gcd) + "/" + (den / gcd);
    }

    private int GCD(int a, int b) {
        return b == 0 ? a : GCD(b, a % b);
    }
}