<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>

//stack for char
class Solution {
public:
    int calculate(string s) {
        int n = 0;
        int operand = 0;
        stack<variant<int, char>> st;

        for(int i = s.size()-1; i >= 0; i--){
            char it = s[i];


            if(isdigit(it)){
                operand += (pow(10,n)*(it - '0'));
                n++;
            }
            else if(it != ' '){
                if(n != 0){
                    st.push(operand);
                    operand= 0;
                    n = 0;
                }

                if( it == '('){
                    int res = evaluate(st);
                    st.pop();
                    st.push(res);
                }
                else{
                    st.push(it);
                }
            }
        }

        if(n!=0){
            st.push(operand);
        }

        return evaluate(st);
    }

    int evaluate( stack<variant<int, char>>& st){
        
        if(st.empty() || holds_alternative<char>(st.top())){
            st.push(0);
        }

        int res = get<int>(st.top()); st.pop();

        while(!st.empty() && holds_alternative<char>(st.top()) && get<char>(st.top()) != ')'){
            char sign = get<char>(st.top());
            if(sign == '+'){
                st.pop();
                res += get<int>(st.top());
                st.pop();
            }
            else{
                st.pop();
                res -= get<int>(st.top());
                st.pop();
            }
        }
        return res;
    }

};
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