<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 process(vector<string> v) {
        int sol = 0;
        int i = 0;
        while (i < v.size()) {
            if (v[i] == "+") {
                sol = sol + stoi(v[i + 1]);
                i += 2;
            } else if (v[i] == "-") {
                sol = sol - stoi(v[i + 1]);
                i += 2;
            } else {
                sol += stoi(v[i]);
                i++;
            }
        }
        return sol;
    }
    int eval(string s, int i, int j) {
        stack<string> st;
        string num;
        while (i <= j) {
            if (s[i] >= '0' && s[i] <= '9') {
                num.push_back(s[i]);
            } else {
                if(num.size() > 0) {
                    st.push(num);
                    num.clear();
                }
                if (s[i] == ')') {
                    int sol = 0;
                    vector<string> v;
                    while (st.top() != "(") {
                        v.push_back(st.top());
                        st.pop();
                    }
                    st.pop();
                    reverse(v.begin(), v.end());
                    st.push(to_string(process(v)));
                }
                else if(!isspace(s[i])) {
                    string ch;
                    ch.push_back(s[i]);
                    st.push(ch);
                }
            }
            i++;
        }
        return stoi(st.top());
    }
    int calculate(string s) {
        s = "(" + s + ")";
        int n = s.size();
        return eval(s, 0, n - 1);
    }
};
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