basic calculator II

/*
Precedence:
*,/ ->Left to right as per in expression
+,- ->Left to right as per in expression

so,* and division should be handles first.
so we will be handling first * and division so after resolving multiplication and division we are only
having positive and negative numbers so we will store positive and negative numbers in stack and finally
we will add all numbers.
*/
class Solution {
public:
    int calculate(string s) {
        int size= s.size(),i=0,ans=0;
        char prevOperator='+';
        stack<int> sta;
        while(i<size){
            if(s[i]==' '){   //if space then ignore the character
                i++;
                continue;
            }
            int num=0;
            while(isdigit(s[i]))   //extracting the number
                num = num*10 + (s[i++]-'0');
            if(prevOperator=='+')   //if current sign is positive then push number
                sta.push(num);
            else if(prevOperator=='-') //if sign is negative push -1*num
                sta.push(-num);
            else if(prevOperator=='*'){  //if multiply opertor then multiply current number
                int x=sta.top();         //with top of the stack
                sta.pop();
                sta.push(x*num);
            }
            else if(prevOperator=='/'){  //divide current num with top of the stack
                int x=sta.top();
                sta.pop();
                sta.push(x/num);
            }
            prevOperator=s[i++];
        }
        while(sta.size()){
            ans+=sta.top();
            sta.pop();
        }
        return ans;
    }
};