leetcode 224. 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 {
	int my_atoi(string s)
	{
		int k = s[0] == '-' ? 1: 0;
		int re = 0;
		while (k < s.length())
		{
			re = 10 * re + s[k] - '0';
			k++;
		}
		return s[0] == '-' ? -re : re;
	}
	string my_itoa(int num)
	{
		if (num == 0)
			return "0";
		string s;
		int n = abs(num);
		while (n != 0)
		{
			s.insert(s.begin(), n % 10 + '0');
			n /= 10;
		}
		if (num < 0)
			s.insert(s.begin(), '-');
		return s;
	}
	int cal_parenthes(string s)
	{
		int starter = s[0] == '-' ? 1 : 0;
		int k = starter + 1;
		while (k < s.length() && (s[k] -'0'>=0 && s[k] - '9'<=0))
			k++;
		if (k == s.length())
			return my_atoi(s);
		int num1 = my_atoi(string(s.begin(), s.begin() + k));
		int st2 = k + 1; k++;
		while (k < s.length() && (s[k] - '0'>=0 && s[k] -'9'<=0))
			k++;
		int num2 = my_atoi(string(s.begin() + st2, s.begin() + k));
		int re = s[st2-1] == '+' ? num1 + num2 : num1 - num2;
		s.erase(s.begin(), s.begin() + k); 
		s = my_itoa(re) + s;
		return cal_parenthes(s);
	}
public:
	int calculate(string s) {
		int p = s.find(' ');
		while (p != string::npos)
		{
			s.erase(s.begin() + p, s.begin() + p + 1);
			p = s.find(' ');
		}
		int pos = s.find(')');
		while (pos != string::npos)
		{
			int pos1 = pos - 1;
			while (s[pos1] != '(')
				pos1--;
			int re = cal_parenthes(string(s.begin() + pos1 + 1, s.begin() + pos));
			string ss = my_itoa(re);
			if (re < 0)
			{
				if (pos1 - 1 > 0)
				{
					if (s[pos1 - 1] == '-')
					{
						s.erase(s.begin() + pos1 - 1, s.begin() + pos + 1);
						s.insert(s.begin() + pos1 - 1, ss.begin(), ss.end());
						s[pos1 - 1] = '+';
					}
					else
					{
						s.erase(s.begin() + pos1 - 1, s.begin() + pos + 1);
						s.insert(s.begin() + pos1 - 1, ss.begin(), ss.end());
					}
				}
				else
				{
					s.erase(s.begin() , s.begin() + pos + 1);
					s.insert(s.begin() , ss.begin(), ss.end());
				}
			}
			else
			{
				s.erase(s.begin() + pos1 , s.begin() + pos + 1);
				s.insert(s.begin() + pos1 , ss.begin(), ss.end());
			}
			pos = s.find(')');
		}
		return cal_parenthes(s);
	}
};

accepted