<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) {
        vector<pair<int,int>> ts;
        for(int i = 0;i < s.size();i++){
            if(s[i] == ' ')continue;
            else if(s[i] == '-' && (i == 0 || ts.back().second == '(' )){
                ts.push_back({0,0});
                ts.push_back({0,'-'});
            }
            else if(s[i] >= '0' && s[i] <= '9'){
                int res = 0;
                while(i < s.size() && s[i] >= '0' && s[i] <= '9'){
                    res *= 10;
                    res += s[i] - '0';
                    i++;
                }
                ts.push_back({res, 0});
                i--;
            }
            else ts.push_back({0, s[i]});
        }
        stack<pair<int,int>> ops;
        vector<pair<int,int>> rpn;
        for(auto &t : ts){
            if(t.second == 0) rpn.push_back(t);
            else if(t.second == '+' || t.second == '-'){
                while(!ops.empty() && (ops.top().second == '+' || ops.top().second == '-')){
                    rpn.push_back(ops.top());
                    ops.pop();
                }
                ops.push(t);
            }
            else if(t.second == '(')ops.push(t);
            else{
                while(!ops.empty() && ops.top().second != '('){
                    rpn.push_back(ops.top());
                    ops.pop();
                }
                ops.pop();
            }
        }
        while(!ops.empty()){
            rpn.push_back(ops.top());ops.pop();
        }
        stack<int> o;


        for(auto &[n,t] :rpn){
            if(t == 0) o.push(n);
            else{
                int op2 = o.top(); o.pop();
                int op1 = o.top(); o.pop();
                if(t == '+') o.push(op1 + op2);
                else o.push(op1 - op2);
            }
        }
        return o.top();

    }

};
21556