<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 getNum(const string& s, int& i) {
        int num = 0;
        while (i < s.size() && isdigit(s[i])) {
            num = num * 10 + (s[i] - '0');
            i++;
        }
        i--;
        return num;
    }

    int calculate(string s) {
        char lastOp = 0;
        int lastNum = 0;
        int i = 0;

        while (i < s.size()) {
            if (isdigit(s[i])) {
                int num = getNum(s, i);

                if (lastOp == '+') {
                    lastNum += num;
                } else if (lastOp == '-') {
                    lastNum -= num;
                } else {
                    lastNum = num;
                }
            } else if (s[i] == '+') {
                lastOp = '+';
            } else if (s[i] == '-') {
                lastOp = '-';
            } else if (s[i] == '(') {
                int counter = 1;
                int ind = i + 1;

                while (counter > 0) {
                    if (s[ind] == '(')
                        counter++;
                    else if (s[ind] == ')')
                        counter--;
                    ind++;
                }

                int num = calculate(s.substr(i + 1, ind - i - 2));

                if (lastOp == '+') {
                    lastNum += num;
                } else if (lastOp == '-') {
                    lastNum -= num;
                } else {
                    lastNum = num;
                }

                i = ind - 1;
            }
            i++;
        }

        return lastNum;
    }
};
auto init = []() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    return 0;
}();
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