<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.length();
        cout<<n<<endl;
        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(s[i]!=')'){
                string temp;
                int j=i;
                while(j<n&&s[j]!=')'&&s[j]!='('&&s[j]!='+'&&s[j]!='-'){
                    if(s[j]==' ') j++;
                    else{
                    temp.push_back(s[j]);
                    j++;
                    }
                }
               i=j-1;
               st.push(temp);
            }
            else{
                int ans=0;
                while(st.size()>0&&st.top()!="("){
                    string temp=st.top();
                    st.pop();
                    string ans1=st.top();
                    st.pop();
                    if(ans1=="("){
                        ans+=stoi(temp);
                        break;
                    }
                    else{
                        if(ans1=="+"){
                            ans+=stoi(temp);
                        }
                        else{
                            ans-=stoi(temp);
                        }
                    }
                }
                if(st.size()>0&&st.top()=="(") st.pop();
              st.push(to_string(ans));
            }
        }
        int ans=0;
        while(st.size()>0){
            if(st.size()<2){
                string ans2=st.top();
                ans+=stoi(ans2);
                st.pop();
            }
            else{
            string ans1=st.top();
            st.pop();
            string temp=st.top();
            st.pop();
            if(temp=="+"){
                ans+=stoi(ans1);
            }
            else{
                ans-=stoi(ans1);
            }
            }
        }
        // while(st.size()>0){
        //     string p=st.top();
        //     cout<<p<<endl;
        //     st.pop();
        // }
        return ans;
    }
};
23431