Basic Calculator II

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) {
        stack<int> num;
        stack<char> symbol;
        symbol.push('+');
        int n;
        n=s.length();
        int right;
        if(n<1) return -1;
        int i;
        for(i=0;i<n;)
        {
            if(s[i]=='+'||s[i]=='-'||s[i]=='*'||s[i]=='/')
            {
                symbol.push(s[i]);
                i++;
            }
            else if(s[i]==' ')
                    i++;
                 else
                 {
                     int right=0;
                     while(i<n&&s[i]>='0'&&s[i]<='9')
                     {
                         right=right*10+(s[i]-'0');
                         i++;
                     }
                     if((!symbol.empty())&&(symbol.top()=='*'||symbol.top()=='/'))
                     {
                         right=cal_helper(num.top(),right,symbol.top());
                         num.pop();
                         symbol.pop();
                     }
                     num.push(right);
                 }
        }
        int res=0;
        char sym;
        int left;
        while(!symbol.empty())
        {
            left=num.top();
            sym=symbol.top();
            if(sym=='-')
              left=-left;
            num.pop();
            symbol.pop();
            res+=left;
        }
        return res;
    }
private:
    int cal_helper(int left, int right, char s)
    {
        
        switch(s)
        {
        case '*':return left*right;
        case '/':return left/right;
        default:return 0;
        }
        
    }
};