<p>Given a string <code>s</code> representing a valid expression, implement a basic calculator to evaluate it, and return <em>the result of the evaluation</em>.</p>

<p><strong>Note:</strong> You are <strong>not</strong> allowed to use any built-in function which evaluates strings as mathematical expressions, such as <code>eval()</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;1 + 1&quot;
<strong>Output:</strong> 2
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot; 2-1 + 2 &quot;
<strong>Output:</strong> 3
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;(1+(4+5+2)-3)+(6+8)&quot;
<strong>Output:</strong> 23
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 3 * 10<sup>5</sup></code></li>
	<li><code>s</code> consists of digits, <code>&#39;+&#39;</code>, <code>&#39;-&#39;</code>, <code>&#39;(&#39;</code>, <code>&#39;)&#39;</code>, and <code>&#39; &#39;</code>.</li>
	<li><code>s</code> represents a valid expression.</li>
	<li><code>&#39;+&#39;</code> is <strong>not</strong> used as a unary operation (i.e., <code>&quot;+1&quot;</code> and <code>&quot;+(2 + 3)&quot;</code> is invalid).</li>
	<li><code>&#39;-&#39;</code> could be used as a unary operation (i.e., <code>&quot;-1&quot;</code> and <code>&quot;-(2 + 3)&quot;</code> is valid).</li>
	<li>There will be no two consecutive operators in the input.</li>
	<li>Every number and running calculation will fit in a signed 32-bit integer.</li>
</ul>

class Solution {
public:
int make_oper(string oper, int first, int second)
{
    if (oper == "+") return first + second;
    else return first - second;
}
int calculate(string s)
{
    stack<string> calc;

    int i = 0, res = 0;
    string d;
    while (i < s.size())
    {
        if (s[i] == ' ')
        {
            i++;
            continue;
        }

        if (s[i] == ')')
        {
            vector<string> now;
            if (d.size()) calc.push(d);
            d.clear();
            while (calc.top() != "(")
            {
                now.push_back(calc.top());
                calc.pop();
            }
            calc.pop();
            reverse(now.begin(), now.end());
            int cur = 0;
            string f = "-";
            for (int j = 0; j < now.size(); ++j)
            {
                if (now[j] == "-" || now[j] == "+")
                {
                    if (j == 0)
                    {
                        j++;
                        cur -= (stoi(now[j]));
                    }
                    else
                    {
                        j++;
                        if (f == "-")
                        {
                            cur = make_oper(now[j - 1], cur, stoi(now[j]));
                        }
                        else
                        {
                            
                            cur += make_oper(now[j - 1], stoi(f), stoi(now[j]));
                        }
                        f = "-";
                    }
                }
                else
                {
                    f = now[j];
                }
            }
            if (f != "-") cur = stoi(f);
            i++;
            calc.push(to_string(cur));
        }
        else if (isdigit(s[i]))
        {
            d += s[i];
            i++;
        }
        else
        {
            if(d.size())
            calc.push(d);
            string t;
            t += s[i];
            calc.push(t);
            i++;
            d.clear();
        }

    }
    if (d.size()) calc.push(d);
    if (calc.size() > 1)
    {
        vector<string> t;
        while (calc.size())
        {
            t.push_back(calc.top());
            calc.pop();
        }
        reverse(t.begin(), t.end());
        string f = "-";
        for (int i = 0; i < t.size(); ++i)
        {
            if (t[i] == "-" || t[i] == "+")
            {
                if (i == 0)
                {
                    i++;
                    res -= stoi(t[i]);
                }

                else if (f == "-")
                {
                    res = make_oper(t[i], res, stoi(t[i + 1]));
                    i++;
                }
                else
                {
                    res += make_oper(t[i], stoi(f), stoi(t[i + 1]));
                    i++;
                }
                f = "-";
            }
            else
            {
                f = t[i];
            }
        }
    }
    else
    {
        res = stoi(calc.top());
    }
    return res;
}
};
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