<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) {
        string str="";
        for(int i=0;i<s.size();i++){
            if(s[i]!=' ')
            str.push_back(s[i]);
        }
        s=str;
        stack<string> st;
        s="("+s+")";
        string t="";
        for(int i=0;i<s.size();i++){
            if(s[i]=='('||s[i]=='+'||s[i]=='-'){
                string k="";
                k.push_back(s[i]);
                if(t!=""){
                    st.push(t);
                    t="";
                }
                st.push(k);
            }else if(s[i]>='0'&&s[i]<='9'){
                t.push_back(s[i]);
            }else{
                int ans=0;
                if(t!=""){
                    st.push(t);
                    t="";
                }
                // string srk="";
                // while(!st.empty()){
                //     srk+=st.top();
                //     st.pop();
                // }
                // reverse(srk.begin(),srk.end());
                // cout<<srk<<endl;
                while(st.top()!="("){

                    string s1=st.top();
                    st.pop();
                    if(st.top()=="+"||st.top()=="-"){
                        s1=st.top()+s1;
                        st.pop();
                    }
                    ans+=stoi(s1);
                }
                st.pop();
                if(ans<0&&st.size()>=1&&st.top()=="-"){
                    st.pop();
                    st.push("+");
                    st.push(to_string(abs(ans)));
                }else if(ans<0&&st.size()>=1&&st.top()=="+"){
                    st.pop();
                    st.push("-");
                    st.push(to_string(abs(ans)));

                }
                else{
                    st.push(to_string(ans));
                }
                
            }
        }
        
        return stoi(st.top());
        // return 10;
        
    }
};
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