<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) {
        vector<pair<bool, pair<int, char>>> store;
        string curr_num_str = "";
        bool num_valid = 0;
        int curr_num = 0;
        for (char c : s) {
            if (isdigit(c)) {
                curr_num_str += c;
                continue;
            }
            num_valid = 0;
            if (curr_num_str != "") {
                curr_num = stoi(curr_num_str);
                curr_num_str = "";
                num_valid = 1;
            }
            if (num_valid || c == ')') {
                while (!store.empty()) {
                    if (store[size(store)-1].first) {
                        if (store[size(store)-1].second.second == '(') {
                            if (c != ')') {
                                break;
                            }
                            c = ' ';
                        }
                        if (store[size(store)-1].second.second == '-' && num_valid) {
                            curr_num = -curr_num;
                        }
                    } else {
                        if (!num_valid) {
                            num_valid = 1;
                            curr_num = store[size(store)-1].second.first;
                        } else {
                            curr_num = store[size(store)-1].second.first + curr_num;
                        }
                    }
                    store.pop_back();
                }
            }
            if (num_valid) {
                store.push_back(pair<bool, pair<int, char>>(0, pair<int, char>(curr_num, 'x')));
            } 
            if (c != ' ' && c != ')') {
                store.push_back(pair<bool, pair<int, char>>(1, pair<int, char>(0, c)));
            }
        }
        if (curr_num_str != "") {
            curr_num = stoi(curr_num_str);
            store.push_back(pair<bool, pair<int, char>>(0, pair<int, char>(curr_num, ' ')));
            curr_num_str = "";
        }
        num_valid = 0;

        while (!store.empty()) {
            if (store[size(store)-1].first) {
                if (store[size(store)-1].second.second == '-' && num_valid) {
                    curr_num = -curr_num;
                }
            } else {
                if (!num_valid) {
                    num_valid = 1;
                    curr_num = store[size(store)-1].second.first;
                } else {
                    curr_num = store[size(store)-1].second.first + curr_num;
                }
            }
            store.pop_back();
        }
        if (num_valid) store.push_back(pair<bool, pair<int, char>>(0, pair<int, char>(curr_num, ' ')));

        return store[size(store)-1].second.first;
    }
};
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