<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) {
        stack<string> bracket;
        vector<string> given;
        vector<string> reverse;
        string new1="";
        for(int i=0;i<s.size();i++){
            if(s[i]!=' '){
                new1+=s[i];
            }
        }
        s=new1;
        for(int i=0;i<s.size();i++){
            new1="";
            while('0'<=s[i] && s[i]<='9' && i<s.size()){
                new1+=s[i];
                i++;
            }
            if(new1!=""){
                given.push_back(new1);
                if(i==s.size()){
                    break;
                }
                i--;
            }
            else{
                if(s[i]=='-'){
                    if(i==0){
                        given.push_back("0");
                    }
                    else if(s[i-1]=='('){
                        given.push_back("0");
                    }
                }
                given.push_back(s[i]+new1);
            }
        }
        for(int i=0;i<given.size();i++){
            if(given[i]!="(" &&given[i]!=")" &&given[i]!="+" && given[i]!="-" ){
                reverse.push_back(given[i]);
            }
            else if(given[i]=="(" && i!=0){
                bracket.push(given[i-1]);
            }
            else if(given[i]==")"){
                if(!bracket.empty()){
                    reverse.push_back(bracket.top());
                    bracket.pop();
                }
            }
            else if((given[i]=="+" || given[i]=="-") && given[i+1]!="("){
                reverse.push_back(given[i+1]);
                reverse.push_back(given[i]);
                i+=1;
            }
        }
        stack<int> cal;
        for(int i=0;i<reverse.size();i++){
            if(reverse[i]!= "+" && reverse[i]!="-"){
                cal.push(stoi(reverse[i]));
            }
            if(reverse[i]=="+"){
                int x=0;
                int y=0;

                    x=cal.top();
                    cal.pop();
                    y=cal.top();
                    cal.pop();
                cal.push(x+y);
            }
            else if(reverse[i]=="-"){
                int x=0;
                int y=0;
                    x=cal.top();
                    cal.pop();
                    y=cal.top();
                    cal.pop();
                cal.push(-x+y);
            }
        }
        return cal.top();
    }
};
51556