<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<int> values;
        stack<char> ops;

        int current_value = 0;
        bool negate = false;
        bool last_token_was_op = true;
        for (int i = 0; i < s.length(); i++) {
            if (s[i] == ' ') continue;

            if (s[i] >= '0' && s[i] <= '9') {
                while (i < s.length() && s[i] >= '0' && s[i] <= '9') {
                    current_value *= 10;
                    current_value += s[i] - '0';
                    i++;
                }
                // if (negate) {
                //     current_value *= -1;
                //     negate = false;
                // }
                values.push(current_value);
                current_value = 0;
                i--;
                last_token_was_op = false;
            } else if (last_token_was_op && s[i] == '-') {
                values.push(0);
                ops.push('-');
            } else if (s[i] == '+' || s[i] == '-') {
                while (!ops.empty() && ops.top() != '(') {
                    int b = values.top(); values.pop();
                    int a = values.top(); values.pop();
                    char op = ops.top(); ops.pop();

                    if (op == '+') values.push(a + b);
                    else if (op == '-') values.push(a - b);
                }

                ops.push(s[i]);
                last_token_was_op = true;
            } else if (s[i] == '(') {
                ops.push(s[i]);
                last_token_was_op = true;
            } else if (s[i] == ')') {
                while (!ops.empty() && ops.top() != '(') {
                    int b = values.top(); values.pop();
                    int a = values.top(); values.pop();
                    char op = ops.top(); ops.pop();

                    if (op == '+') values.push(a + b);
                    else if (op == '-') values.push(a - b);
                }

                // if (negate) {
                //     int v = values.top(); values.pop();
                //     values.push(-v);
                //     negate = false;
                // }

                ops.pop();
                last_token_was_op = false;
            }
        }

        while (!ops.empty()) {
            int b = values.top(); values.pop();
            int a = values.top(); values.pop();
            char op = ops.top(); ops.pop();

            if (op == '+') values.push(a + b);
            else if (op == '-') values.push(a - b);
        }

        return values.top();
    }
};
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