<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 solve(string &s,int l, int r){
        int res = 0;
        for(int i=l;i<=r;i++){
            if((s[i]=='-'||s[i]=='+')){
                int m = (s[i]=='-')? -1 : 1; i++;
                while(i<=r&&s[i]==' ') i++;
                if(s[i]=='('){
                    int j = i+1; int ct=1;
                    while(j<=r&&ct!=0){
                        if(s[j]==')') ct--;
                        else if(s[j]=='(') ct++;
                        j++;
                    }
                    res += m*solve(s,i+1,j-2);
                    i = j-1; 
                }
                else {
                    while(i<=r&&s[i]==' ') i++;
                    int j = i;
                     while(j<=r&&s[j]<='9'&&s[j]>='0') j++;
                     res += m*stoi(s.substr(i,j-i));
                     i=j-1;
                }
            }
            else if(s[i]=='('){
                int j = i+1; int ct = 1;
                while(j<=r&&ct!=0){
                    if(s[j]=='(') ct++;
                    else if(s[j]==')') ct--;
                    j++;
                }
                res += solve(s,i+1,j-2);
                i=j-1; 
            }
            else if(s[i]!=' ') {
                int f = (s[i]=='-')? -1 : 1;
                if(s[i]=='+'||s[i]=='-') i++;
                while(i<=r&&s[i]==' ') i++;
                int j = i;
                while(j<=r&&s[j]<='9'&&s[j]>='0') j++;
                res += f*stoi(s.substr(i,j-i));
                i = j-1;
            }
        }
        return res;
    }
    int calculate(string s) {
        ios::sync_with_stdio(0); cin.tie(0);
        return solve(s,0,s.size()-1);
    }
};
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