<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> st;
        int n = s.size();
        for(int i = 0; i < n; i++) {
            if(s[i] == ' ') continue;
            else if(s[i] == '(') st.push("(");
            else if(s[i] == '+') st.push("+");
            else if(s[i] == '-') st.push("-");
            else if(isdigit(s[i])) {
                int j = i + 1;
                while(j < n && isdigit(s[j])) j++;
                st.push(s.substr(i, j - i));
                i = j - 1;
            }
            else {
                double val = 0, lastAdd = 0;
                while(!(st.top() == "(")) {
                    string str = st.top();
                    st.pop();
                    if(str != "+" && str != "-") {
                        val += stoi(str);
                        lastAdd = stoi(str);
                    }
                    else if(str == "-") val -= (2*lastAdd);
                }
                st.pop();
                st.push(to_string(val));
            }
        }

        double val = 0, lastAdd = 0;
        while(!st.empty()) {
            string str = st.top();
            st.pop();
            if(str != "+" && str != "-") {
                val += stoi(str);
                lastAdd = stoi(str);
            }
            else if(str == "-") val -= (2*lastAdd);
        }

        return val;
    }
};
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