<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 n = s.size();

        stack<string> st;
        string tmpNum = "";
        int cnt = 0;
        int tmpCnt = 0;
        int tmpA = 0;
        int tmpB = 0;
        bool isNeg = false;
        bool isBasket = false;
        char opera;

        for(int i = 0; i < n; i++)
        {
            if(s[i] != ' ')
            {
                if(s[i] == ')')
                {
                    isBasket = false;
                    isNeg = false;
                    while(true)
                    {
                        tmpCnt = 0;
                        tmpA = stoi(st.top());
                        st.pop();

                        if(st.top() == "(")
                        {
                            st.pop();
                            if(st.size() && st.top() == "-")
                            {
                                isNeg = true;
                                st.pop();
                            }

                            if(isNeg)
                                st.push(to_string(-1 * tmpA));
                            else
                                st.push(to_string(tmpA));

                            break;
                        }

                        opera = st.top()[0];
                        st.pop();
                        tmpB = stoi(st.top());
                        st.pop();

                        if(opera == '+')
                        {
                            tmpCnt = tmpA + tmpB;
                        }
                        else
                        {
                            tmpCnt = tmpB - tmpA;
                        }

                        //printf("%d\n", tmpCnt);

                        if(st.top() == "(")
                        {
                            st.pop();
                            isBasket = true;
                            if(st.size() && st.top() == "-")
                            {
                                isNeg = true;
                                st.pop();
                            }
                        }

                        if(isNeg)
                            st.push(to_string(-1 * tmpCnt));
                        else
                            st.push(to_string(tmpCnt));

                        //printf("%d %c %d\n", tmpA, opera, tmpB);

                        if(isBasket)
                            break;
                    }

                    //printf("%s\n", st.top().c_str());
                }
                else
                {
                    if(s[i] == '+' || s[i] == '(')
                    {
                        st.push(string(1, s[i]));
                    }
                    else
                    {
                        tmpNum = "";
                        isNeg = false;

                        if(s[i] == '-')
                        {
                            isNeg = true;
                            tmpNum += '-';
                            i++;
                        }

                        while(s[i] >= '0' && s[i] <= '9' || s[i] == ' ')
                        {
                            if(s[i] != ' ')
                                tmpNum += s[i];
                            i++;

                            if(i == n)
                                break;
                        }
                        i--;
                        if(isNeg /*&& tmpNum != "-"*/ && st.size() && st.top() != "(")
                            st.push("+");
                        st.push(tmpNum);
                    }
                }
            }
        }

        while(st.size() > 1)
        {
            tmpCnt = 0;
            tmpA = stoi(st.top());
            st.pop();
            opera = st.top()[0];
            st.pop();
            tmpB = stoi(st.top());
            st.pop();

            //printf("%d %c %d\n", tmpA, opera, tmpB);

            if(opera == '+')
            {
                tmpCnt = tmpA + tmpB;
            }
            else
            {
                tmpCnt = tmpB - tmpA;
            }

            //if(!st.size())
            //    break;

            st.push(to_string(tmpCnt));
        }

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