227. Basic Calculator II(unsolved)

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:
“3+2*2” = 7
” 3/2 ” = 1
” 3+5 / 2 ” = 5
Note: Do not use the eval built-in library function.

class Solution {
public:
    int calculate(string s) {
        int result=0,num=0;
        char sign='+';
        stack<int> st;
        for(int i=0;i<s.size() ;i++){
          if(isdigit(s[i])){
              num=num*10+s[i]-'0';
          }
          if((!isdigit(s[i])&&s[i]!=' ' )|| i == s.size() - 1){
              if(sign=='+') st.push(num);
              else if(sign=='-') st.push(-num);
              else if(sign=='*'){
                  int tmp=st.top()*num;
                  st.pop();
                  st.push(tmp);                  
              }
              else if(sign=='/'){
                  int tmp=st.top()/num;
                  st.pop();
                  st.push(tmp);                  
              }
              num=0;
              sign=s[i];
          }                        
        }
        while(!st.empty()){
            result+=st.top();
            st.pop();         
        }
        return result;    
    }
};