<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 calculate(string s) {
        string ss;
        for (char c : s) {
            if (c != ' ') {
                ss += c;
            }
        }

        if (ss[0] == '-') {
            ss = '0' + ss;
        }

        vector<string> arr;
        for (int i = 0; i < ss.size(); i++) {
            if (ss[i] == '(' || ss[i] == '+' || ss[i] == '-') {
                string t = "";
                arr.push_back(t + ss[i]);
            } else if (ss[i] == ')') {
                vector<string> temp;
                while (arr.back() != "(") {
                    temp.push_back(arr.back());
                    arr.pop_back();
                }
                arr.pop_back();
                std::reverse(temp.begin(), temp.end());
                arr.push_back(helper(temp));
            } else {
                if (arr.empty() || arr.back()[0] == '+' ||
                    arr.back()[0] == '-' || arr.back()[0] == '(') {
                    string t = "";
                    arr.push_back(t + ss[i]);
                } else {
                    arr.back() += ss[i];
                }
            }
            // for (auto a : arr) {
            //     cout << a;
            // }
            // cout << "\n";
        }
        return stoi(helper(arr));
    }

    string helper(vector<string>& input) {
        if (input.size() == 1) {
            return input[0];
        }

        if (input[0] == "-") {
            input.insert(input.begin(), "0");
        }

        vector<string> stack;
        int i = 0;
        while (i < input.size()) {
            if (input[i] == "+") {
                int res = stoi(stack.back()) + stoi(input[i + 1]);
                stack.pop_back();
                stack.push_back(to_string(res));
                i += 2;
            } else if (input[i] == "-") {
                int res = stoi(stack.back()) - stoi(input[i + 1]);
                stack.pop_back();
                stack.push_back(to_string(res));
                i += 2;
            } else {
                stack.push_back(input[i]);
                i += 1;
            }
        }
        return stack[0];
    }
};
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