<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) {
        int res = 0; 
        int cur = 0; 
        bool processing_num = false; 
        bool is_neg = false; 
        for (int i = 0; i < s.length(); i++) {
            char c = s[i]; 

            if (c == '(') {
                int start = i; 
                int len = 0; 
                int cnt = 1; 
                while (cnt != 0) {
                    i++; 
                    len++;
                    if (s[i] == ')')
                        cnt--; 
                    if (s[i] == '(')
                        cnt++;
                }
                int subexpr = calculate(s.substr(start + 1, len - 1)); 
                res += is_neg ? -subexpr : subexpr; 
                continue; 
            }

            if ('0' <= c && c <= '9') {
                if (!processing_num)
                    processing_num = true; 
                cur = cur * 10 + (c - '0'); 
            } else {
                if (processing_num)
                    processing_num = false;

                if (cur > 0) {
                    res += is_neg ? -cur : cur; 
                    cur = 0; 
                }

                if (c == '-') {
                    is_neg = true;  
                    continue; 
                }

                if (c == ' ')
                    continue; 

                if (is_neg)
                    is_neg = false; 

                if (c == '+')
                    continue; 
            } 

            if (i == s.length() - 1)  
                res += is_neg ? -cur : cur; 
        }
        return res;
    }
};
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