<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) {
        stack<string> memo;

        for (int i = 0; i < s.size(); ++i) {
            char currChar = s[i];
            if (currChar == ')'){
                cleanStack(memo);
            }
            else{
                if (currChar == ' '){
                    continue;
                }
                // cout << "Added: " << currChar << endl;
                memo.push(string() + currChar);
            }
        }

        cleanStack(memo);
        // cout << "End of Algo. Stack Top:  " << memo.top() << endl;
        return stoi(memo.top());
    }

    void cleanStack(stack<string>& memo){
        int currSum = 0;
        int localSum = 0;
        int power = 0;

        while(!memo.empty() && memo.top() != "("){
            string currStr = memo.top();
            memo.pop();

            if (currStr == "+" || currStr == "-"){
                
                localSum += currStr == "-" ? currSum * -1 : currSum;
                // cout << "Adding to Local Sum... Curr Sum:  " << currSum << ", Local Sum: " << localSum << endl;
                currSum = 0;
                power = 0;
            } else {
                int ogCurrSum = currSum;
                currSum += stoi(currStr) * pow(10, power);
                power++;
                // cout << "Adding to Curr Sum... Was " << ogCurrSum << ", Now is: " << currSum << ", Power++: " << power << endl; 
            }
        }

        localSum += currSum;
        // cout << "Adding to Local Sum... Curr Sum:  " << currSum << ", Local Sum: " << localSum << endl;

        if (!memo.empty()) memo.pop();
        memo.push(to_string(localSum));     
    }
};
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