Leetcode-592: fraction addition and subtraction

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Given a string representing the addition and subtraction of fractions expression , You need to return a string calculation .

This result should be an irreducible score , The simplest fraction . If the final result is an integer , for example 2, You need to convert it into fractions , Its denominator is 1. So in the above example , 2 Should be converted to 2/1.

Example 1:

 Input : expression = "-1/2+1/2"
 Output : "0/1"

Example 2:

 Input : expression = "-1/2+1/2+1/3"
 Output : "1/3"

Example 3:

 Input : expression = "1/3-1/2"
 Output : "-1/6"

Problem solving

Method 1 ： simulation

Reference link

class Solution {
    
public:
    string fractionAddition(string expression) {
    
        long long a=0,b=1;// molecular , The denominator 
        int index=0,n=expression.size();
        while(index<n){
    
            // Read the sign 
            int sign=1;// If no symbols are encountered , Default positive 
            if(expression[index]=='+'||expression[index]=='-'){
    
                sign=expression[index]=='+'?1:-1;
                index++;
            }

            // Read molecules 
            long long a1=0;
            while(isdigit(expression[index])){
    
                a1=a1*10+expression[index]-'0';
                index++;
            }
            index++;// skip '/'

            // Read denominator 
            long long b1=0;
            while(isdigit(expression[index])){
    
                b1=b1*10+expression[index]-'0';
                index++;
            }

            a1=a1*sign;

            a=a*b1+a1*b;
            b=b*b1;
        }
        if(a==0) return "0/1";

        long long g=gcd(abs(a),b);// Get the greatest common divisor 
        return to_string(a/g)+"/"+to_string(b/g);
        

    }
};