Basic Calculator.cpp

// Runtime: 9 ms (Top 58.93%) | Memory: 8.40 MB (Top 79.42%)

class Solution {
public:
   int calculate(string s) {
       stack<pair<int,int>> st; // pair(prev_calc_value , sign before next bracket () )
       
       long long int sum = 0;
       int sign = +1;
       
       for(int i = 0 ; i < s.size() ; ++i)
       {
           char ch = s[i];
           
           if(isdigit(ch))
           {
               long long int num = 0;
               while(i < s.size() and isdigit(s[i]))
               {
                   num = (num * 10) + s[i] - '0';
                   i++;
               }
               i--; // as for loop also increase i , so if we don't decrease i here a sign will be skipped
               sum += (num * sign);
               sign = +1; // reseting sign
           }
           else if(ch == '(')
           {
               // Saving current state of (sum , sign) in stack
               st.push(make_pair(sum , sign));
               
               // Reseting sum and sign for inner bracket calculation
               sum = 0; 
               sign = +1;
           }
           else if(ch == ')')
           {
               sum = st.top().first + (st.top().second * sum);
               st.pop();
           }
           else if(ch == '-')
           {
               // toggle sign
               sign = (-1 * sign);
           }
       }
       return sum;
   }
};