<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;
        vector<string> ans;

        //обратная польская запись в стеке
        s += ' ';
        string strV = "";
        char prev = ' ';
        for(char ch: s) {
            if(isdigit(ch)) {
                strV += ch;
                prev = ch;
                continue;
            }

            if(strV != "") {
                ans.push_back(strV);
                strV = "";
            }

            if(ch == ' ') continue;

            if(ch == '(') {
                st.push("(");
            } else if(ch == ')') {
                while(!st.empty() && st.top() != "(") {
                    ans.push_back(st.top());
                    st.pop();
                }
                if(!st.empty() && st.top() == "(") st.pop();
            } else if(ch == '+') {
                while(!st.empty() && (st.top() == "+" || st.top() == "-" )) {
                    ans.push_back(st.top());
                    st.pop();
                }
                st.push("+");
            } else if(ch == '-') {
                if(!isdigit(prev) && prev != ')') {
                    ans.push_back("0");
                } else {
                    while(!st.empty() && (st.top() == "+" || st.top() == "-" )) {
                        ans.push_back(st.top());
                        st.pop();
                    }
                }
                st.push("-");
            }                

            prev = ch;
        }

        while(!st.empty()) {
            string v = st.top();
            st.pop();
            ans.push_back(v);
        }
        
        //вычислить обратную польскую запись
        for(string v: ans) {
            if(v == "+") {
                int a = stoi(st.top());
                st.pop();
                int b = stoi(st.top());
                st.pop();
                st.push(to_string(a + b));
            } else if(v == "-") {
                int b = stoi(st.top());
                st.pop();
                int a = stoi(st.top());
                st.pop();
                st.push(to_string(a - b));
            } else {
                st.push(v);
            }
        }

        return stoi(st.top());        
    }
};
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