<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;
        s += ")";
        st.push("(");
        for(int i = 0; i<s.size(); 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(s[i] == ')'){
                vector<string> temp;
                while(st.top() != "("){
                    temp.push_back(st.top());
                    st.pop();
                }
                st.pop();
                reverse(temp.begin(),temp.end());
                // for(auto x:temp){cout<<x<<endl;}
                int sum = 0;
                int j = 0;
                if(temp[0]!="-"){
                    sum = stoi(temp[0]);
                    j++;
                }
                while(j<temp.size()){
                    if(temp[j] == "-"){
                        int num = stoi(temp[j] + temp[j+1]);
                        j+=2;
                        sum += num;
                    }
                    else if(temp[j] == "+"){
                        int num = stoi(temp[j+1]);
                        j+=2;
                        sum += num;
                    }
                    else{j+=2;}
                }
                if(st.empty()){
                    if(sum<0){
                        st.push("-");
                        sum = -sum;
                    }
                    st.push(to_string(sum));
                }
                else if(st.top() == "("){
                    if(sum<0){
                        st.push("-");
                        sum = -sum;
                    }
                    st.push(to_string(sum));
                }
                else if(st.top() == "+"){
                    if(sum<0){
                        st.pop();
                        st.push("-");
                        sum = -sum;
                        st.push(to_string(sum));
                    }
                    else{
                        st.push(to_string(sum));
                    }
                }
                else if(st.top() == "-"){
                    // cout<<"here"<<endl;
                    if(sum<0){
                        st.pop();
                        if(st.size()>=2){st.push("+");}
                        sum = -sum;
                        st.push(to_string(sum));
                    }
                    else{
                        st.push(to_string(sum));
                    }
                }

            }
            else{
                int fullnum = 0;
                while(i<s.size()){
                    int num = s[i]-'0';
                    if(num>=0 && num<=9){fullnum = 10*fullnum + num;}
                    else{
                        i--;
                        break;
                    }
                    i++;
                }
                st.push(to_string(fullnum));
            }
        }
        // cout<<st.size()<<endl;
        if(st.size()==1){return stoi(st.top());}
        else{
            // while(!st.empty()){cout<<st.top()<<endl;
            // st.pop();}
            return -1*stoi(st.top());
            return -1;
            }
    }
};
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