<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>

#include <iostream>
#include <stack>
#include <string>
#include <vector>
using namespace std;
class Solution {
public:
    vector<string> createTokens(string s) {
        vector<string> tokens;
        int start = 0, end = 0;
        if (s[0] != '+' && s[0] != '-') {
            tokens.push_back("+");
        }

        while (end < s.length()) {
            if (s[start] == ' ') {
                start++;
                end++;
            }
            else if (s[start] == '+' || s[start] == '-' || s[start] == '(' || s[start] == ')') {
                tokens.push_back({s[start]});
                start++;
                end++;
            }
            else if (s[end] == '+' || s[end] == '-' || s[end] == '(' || s[end] == ')' || s[end] == ' ') {
                if (tokens.back() == "+") {
                    tokens.pop_back();
                    tokens.push_back("+" + s.substr(start, end - start));
                }
                else if (tokens.back() == "-") {
                    tokens.pop_back();
                    tokens.push_back("-" + s.substr(start, end - start));
                }
                else {
                    tokens.push_back("+" + s.substr(start, end - start));
                }
                start = end;
            }
            else {
                end++;
            }
        }
        if (start < s.length()) {
            if (tokens.back() == "+") {
                tokens.pop_back();
                tokens.push_back("+" + s.substr(start, end - start));
            }
            else if (tokens.back() == "-") {
                tokens.pop_back();
                tokens.push_back("-" + s.substr(start, end - start));
            }
            else {
                tokens.push_back("+" + s.substr(start, end - start));
            }
        }

        return tokens;
    }

    int calculate(string s) {
        vector<string> tokens = createTokens(s);

        for (auto token : tokens) {
            cout << token << ", ";
        }
        cout << "\n";

        stack<bool> isAdd;
        isAdd.push(true);
        int sum = 0;
        for (string token : tokens) {
            bool isToggle;
            if (token.length() > 1) {
                sum += isAdd.top() ? stoi(token) : 0 - stoi(token);
            }
            else if (token == "+") {
                isToggle = false;
            }
            else if (token == "-") {
                isToggle = true;
            }
            else if (token == "(") {
                isAdd.push(isAdd.top() ^ isToggle);
            }
            else if (token == ")") {
                isAdd.pop();
            }
        }

        return sum;
    }
};
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