Basic Calculator II (Stack)

Title: https://leetcode-cn.com/problems/basic-calculator-ii/
implement a basic calculator to calculate the value of a simple string expression
The string expression contains only non-negative integers, +,-, *, / four operators and spaces. Integer division preserves only the integer part.

class Solution {
public:
    int calculate(string s) {
        char c = '+';
        int n = s.length();
        long long tmp = 0;
        stack<long long> st;
        for(int i = 0;i < n;i++) {
            if(s[i] == ' ') continue;
            if('0'<=s[i]&&s[i]<='9') {
                tmp = 0;
                while(i<n&&'0'<=s[i]&&s[i]<='9')
                    tmp = tmp*10+s[i]-'0',i++;
                i--;
                if(c=='*') st.top() *= tmp;
                else if(c=='/') st.top() /= tmp;
                else if(c=='-') st.push(-1*tmp);
                else st.push(tmp);
            }
            else c = s[i];
        }
        long long res = 0;
        while(!st.empty()) res += st.top(),st.pop();
        return res;
    }
};