<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) {
    std::vector<int> bidx;
    std::vector<int> sign = {1, 1};
    int n = s.length();
    int i = 0, res = 0;
    while (i < n) {
        if (s[i] >= '0' && s[i] <= '9') {
            int tmp = 0;
            while (s[i] >= '0' && s[i] <= '9') {
                tmp = tmp * 10 + (s[i] - '0');
                i++;
            }
            res += sign.back() * tmp;
            sign.pop_back();
            i--;
        } else if (s[i] == '+') {
            sign.push_back(sign.back());
        } else if (s[i] == '-') {
            sign.push_back(-sign.back());
        } else if (s[i] == ' ') {
        } else if (s[i] == '(') {
            int cnt = 1;
            int j = i;
            while (cnt > 0 && ++j < n) {
                if (s[j] == '(') {
                    cnt++;
                } else if (s[j] == ')') {
                    cnt--;
                }
            }
            res += sign.back() * calculate(s.substr(i + 1, j - i - 1));
            sign.pop_back();
            i = j;
        }
        i++;
    }

    return res;
}
};
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