basic caculate ii, C++ (gcc)

basic caculate ii

// Calculate II
// The expression string may contain +, -, *, /, and empty spaces.  
// Input: "3+5 / 2 " = 5

#include <iostream>
#include <stack>
using namespace std;

int calculate2(string s) {
    long res = 0; 
    long n = 0; 
    char sgn = '+';
    stack<long> stack; 
    for(int i=0; i<s.size(); i++){
        char c = s[i];
        if(isdigit(c))
            n = n*10+ c - '0';

if(!isdigit(c) && c !=' ' || i == s.size()-1 ){// ops or last element 
            switch(sgn){
                case '*':
                    stack.top() *= n;
                    break;
                case '/':
                    stack.top() /= n;
                    break;
                case '+':
                    stack.push(n);
                    break;
                case '-':
                    stack.push(-n);
                    break;
            }
            n = 0; sgn = c;  // 
        }
    }
    while(stack.size()){
        res += stack.top();
        stack.pop();
    }
    return res; 
}

int main(){
    string s = "3+5 / 2 "; 
    cout << s << " = " << calculate2(s) << endl; 
}


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