basic caculate i, C++ (gcc)

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// Calculate I
// The expression string may contain "(", ")", +, -, no-negative integers and empty spaces.  
// Input: "(1+(4+5+2)-3)+(6+8)"
// Output: 23

#include <iostream>
#include <stack>

using namespace std;

int calculate_1(string s) {
   stack<long> stack; 
    long n = 0; 
    long sgn = 1; 
    long res = 0; 
    for(int i=0; i<s.size(); i++){
        char c = s[i];
        if(isdigit(c)){
            n = n*10 + c - '0';
        }
        else if (c == '+' ){
            res += n * sgn; 
            n = 0; 
            sgn = 1; 
        }
        else if (c == '-' ){
            res += n * sgn; 
            n = 0; 
            sgn = -1; 
        }
        else if (c == '(' ){ // push 
            stack.push(res); 
            stack.push(sgn);
            res = 0; 
            sgn = 1; 
        }
        else if (c == ')' ){ // pop 
            res += n*sgn; 
            n = 0; 
            sgn = stack.top(); stack.pop();
            res = res *sgn ;  // NOTE!!
            res += stack.top(); stack.pop(); 
        }
    }
    if(n !=0)
        res += n*sgn;

return res; 
}

int main(){
    string s = "(1+(4+5+2)-3)+(6+8)"; 
    cout << s << " = " << calculate_1(s) << endl; 
}

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Compilation time: 0.63 sec, absolute running time: 0.08 sec, cpu time: 0.01 sec, memory peak: 3 Mb, absolute service time: 0,71 sec