<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 helper(vector<string> s) {
    int sum = 0;
    bool add = !(s[0] == "-");
    for (auto &c: s) {
        bool flag = (c[0] == '-' && c.size()>1);
        if (c == " ")
            continue;
        else if (std::all_of(c.begin(), c.end(), ::isdigit) || flag) {
            sum += add ? stoi(c) : stoi(c) * -1;
        } else if (c == "+") {
            add = true;
        } else if (c == "-") {
            add = false;
        }
    }
    return sum;
}

int calculate(string s) {
    stack<string> st;
    int res = 0;
    vector<string> tmp2;
    for (int i = 0; i < s.size(); i++) {
        char c = s[i];
        if (c == ' ')
            continue;
        if (c == ')') {
            vector<string> tmp;
            while (st.top() != "(") {
                tmp.emplace_back(st.top());
                st.pop();
            }
            st.pop();
            reverse(tmp.begin(), tmp.end());
            int sum = helper(tmp);
            cout << "sum=" << sum <<endl;
            st.push(to_string(sum));
            continue;
        }
        if (isdigit(c)) {
            string tmp3;
            int j = i;
            while (isdigit(s[j])){
                tmp3.push_back(s[j]);
                j++;
            }
            i = j - 1;
            st.push(tmp3);
            continue;
        }
        string tmp4;
        tmp4.push_back(c);
        st.push(tmp4);
    }
    while (!st.empty()) {
        tmp2.emplace_back(st.top());
        st.pop();
    }
    reverse(tmp2.begin(), tmp2.end());
    for (auto it =tmp2.begin(); it!=tmp2.end();it++)
        cout << *it<<endl;
    res = helper(tmp2);
    return res;
}
};
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