Basic Calculator

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23

Note: Do not use the eval built-in library function.

class Solution {
public:
    int calculate(string s) {
        int n = s.size();
        if (n < 1)
        {
            return 0;
        }
    
        s = '(' + s + ')';
        n += 2;
        int result = 0;
        stack<char> symbol;
        stack<int> num;
        int value = 0;
        bool numFound = false;

        for (int i = 0; i < n; i++)
        {
            if (s[i] <= '9' && s[i] >= '0')
            {
                numFound = true;
                value = value*10 + s[i] - '0';
                continue;
            }
            else if (numFound)
            {
                num.push(value);
                value = 0;
                numFound = false;
            }
        
            if (s[i] == ')')
            {
                int temp = 0;
                while (true)
                {
                    int v = num.top();
                    num.pop();
                    char w = symbol.top();
                    symbol.pop();
                    if (w == '+')
                    {
                        temp += v;
                    }
                    else if (w == '-')
                    {
                        temp -= v;
                    }
                    else
                    {
                        temp += v;
                        num.push(temp);
                        break;
                    }
                }

                if (i == n-1)
                {
                    result = temp;
                    break;
                }
            }
            else if (s[i] == '+' || s[i] == '-' || s[i] == '(')
            {
                symbol.push(s[i]);
            }
        }

        return result;  
    }
};