Expression evaluation (Stack Sub-pattern)

// + - * / with precedence, no parentheses (Basic Calculator II).
// When a number is finished, apply the operator that came BEFORE it.
int calculate(string s) {
    vector<long long> st;                            // terms waiting to be added
    long long num = 0;
    char op = '+';                                   // pretend the string starts with '+'
    for (int i = 0; i <= (int)s.size(); i++) {
        char c = i < (int)s.size() ? s[i] : '+';     // fake '+' at the end flushes the last number
        if (isdigit(c)) { num = num * 10 + (c - '0'); continue; }
        if (c == ' ') continue;
        if (op == '+') st.push_back(num);
        else if (op == '-') st.push_back(-num);
        else if (op == '*') st.back() *= num;        // * and / bind to the latest term now
        else st.back() /= num;
        op = c;
        num = 0;
    }
    long long sum = 0;
    for (long long t : st) sum += t;                 // only + is left
    return (int)sum;
}