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;      }};