<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:
    bool isOperator(char c) {
        return c == '*' || c == '+' || c == '-' || c == '/';
    }

    static int eval12(int a, int b, string i) {
        if (i == "+")
            return a + b;
        if (i == "*")
            return a * b;
        if (i == "-")
            return a - b;
        if (i == "/")
            return a / b;

        return 0;
    }
    void performOperation(stack<string> &st, string temp) {
        if (st.empty() || st.top() == "(") {
            st.push(temp);
        } else if (isOperator(st.top()[0])) {
            char oper = st.top()[0];
            st.pop();

            // cout<<oper<<endl;

            if (oper == '-' && (st.empty() || st.top() == "(")) {
                st.push(to_string(eval12(0, stoi(temp), string(1, oper))));

            } else {
                string t1 = st.top();
                st.pop();

                // cout<<t1<<" "<<temp;

                st.push(
                    to_string(eval12(stoi(t1), stoi(temp), string(1, oper))));
            }
        }
    }

    int calculate(string s) {
        int n = s.size();
        stack<string> st;
        int i = 0;

        while (i < n) {
            if (s[i] == ' ') {
                i++;
                continue;
            }
            if (s[i] == '(' || isOperator(s[i])) {
                st.push(string(1, s[i]));
                i++;
            } else if (s[i] == ')') {
                string t = st.top();
                st.pop();
                st.pop();
                cout << t << endl;

                performOperation(st, t);

                    // st.push(t);
                i++;
            }

            else {
                string temp = "";

                while (i < n && (s[i] >= '0' && s[i] <= '9')) {
                    temp += s[i++];
                }

                if (st.empty() || st.top() == "(") {
                    st.push(temp);
                } else if (isOperator(st.top()[0])) {
                    char oper = st.top()[0];
                    st.pop();

                    // cout<<oper<<endl;

                    if (oper == '-' && (st.empty() || st.top() == "(")) {
                        cout<<to_string(eval12(0, stoi(temp), string(1,oper)))<<"hello"<<endl;
                        st.push(
                            to_string(eval12(0, stoi(temp), string(1,oper))));

                    } else {
                        string t1 = st.top();
                        st.pop();

                        // cout<<t1<<" "<<temp;

                        st.push(to_string(
                            eval12(stoi(t1), stoi(temp), string(1, oper))));
                    }
                }
            }
        }

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