<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<string> v;
        v.push_back("(");
        string t = "";
        for(int i = 0; i<s.length(); i++) if(s[i] != ' ') t += s[i];
        s = t;
        for(int i = 0;i < s.length(); i++){
            if(s[i] >= '0' && s[i] <= '9'){
                string temp = "";
                while(s[i] >= '0' && s[i] <= '9'){
                    temp += s[i];
                    i++;
                }
                i--;
                v.push_back(temp);
            }
            else if(s[i] != ' '){
                string temp = "";
                temp += s[i];
                v.push_back(temp);
            }
        }    
        v.push_back(")");
        stack<string> st;
        for(int i  = 0; i < v.size(); i++){
            if(v[i] != ")"){
                st.push(v[i]);
            }
            else{
                int temp = 0;
                while(st.top() != "("){
                    int k = stoi(st.top());
                    st.pop();
                    if(st.top() == "+"){
                        temp += k;
                        st.pop();
                    }
                    else if(st.top() == "-"){
                        temp -=k;
                        st.pop();
                    }
                    else temp += k;
                }
                st.pop();
                st.push(to_string(temp));
            }
        }
        return stoi(st.top());
    }
};
43118