basic caculate iii, C++ (gcc)

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// Calculate III
// The expression string may contain (, ) +, -, *, /, and empty spaces.  
// Input: "2*(1+2*5-2/2)+1" = 21

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

long eNum(string& s, int& i) {
    long n = 0;
    while(i < s.length() && isdigit(s[i]))
        n = s[i++] - '0' + 10 * n;
    return n;
}

long experssion(string& s, int& i) {
    vector<long> nums; // you can use either vector or stack
    char op = '+';
    for (; i < s.length() && op != ')'; i++) { // op can take value of +, -, *, / & )
        if (s[i] == ' ') continue; // skip for space
        long n = s[i] == '(' ? experssion(s, ++i) : eNum(s, i);
        switch(op) {
            case '+' : nums.push_back(n); break;
            case '-' : nums.push_back(-n); break;
            case '*' : nums.back() *= n; break;
            case '/' : nums.back() /= n; break;
        }
        op = s[i];
    }

long res = 0; // return results 
    for (int n : nums) res += n;
    return res;
}

int calculate(string s) {
    int i = 0;
    return experssion(s, i);
}

int main(){
    string s = "2*(5+5*2)/3+(6/2+8)"; // = 21
    cout << s << " = " << calculate(s) << endl; 
}

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Compilation time: 0.63 sec, absolute running time: 0.1 sec, cpu time: 0.02 sec, memory peak: 3 Mb, absolute service time: 0,74 sec