<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 {
private:
    string removeSpaces(string s){
        string result = "";
        for (int i = 0; i < s.length(); i++){
            if (s[i]!= ' ') {
                result+=s[i];
            }
        }
        return result;
    }
    vector<string> tokenize(string s){
        vector<string> result;
        string number = "";
        for (int i = 0; i < s.length(); i++){
            if (s[i] == '+' || s[i] == '-' || s[i] == '(' || s[i] == ')') {
                if (number != "") {
                    result.push_back(number);
                }
                result.push_back(std::string{s[i]});
                number = "";
            } else {
                number += s[i];
            }
        }
        if (number != "") {
            result.push_back(number);
        }
        return result;
    }
    string evaluate(vector<string> operation) {
        int i = operation.size() - 1;
        // std::cout << "Operation: [";
        // for(string token: operation) {
        //     std::cout << token << ",";
        // }
        // std::cout << "]";
        while (operation.size() > 1) {
            if (operation[operation.size()- 1] == "-") {
                operation.pop_back();
                operation[operation.size() - 1] = std::to_string(-1 * std::stoi(operation[operation.size() - 1]));
            } else {
                string result = "";
                if (operation[operation.size() - 2 ] == "+") {
                    result = std::to_string(std::stoi(operation[operation.size()-1]) + std::stoi(operation[operation.size()-3]));
                } else {
                    result = std::to_string(std::stoi(operation[operation.size()-1]) - std::stoi(operation[operation.size()-3]));
                }
                operation[operation.size() - 3 ] = result;
                operation.pop_back();
                operation.pop_back();
            }
        }
        // std::cout << " = " << std::string{operation[0]} << std::endl;
        return std::string{operation[0]};
    }
public:
    int calculate(string s) {
        vector<string> tokens = tokenize(removeSpaces(s));
        if (tokens.size() == 1) {
            return std::stoi(tokens[0]);
        }
        stack<string> resolving;
        // std::cout << "[";
        // for(string token: tokens) {
        //     std::cout << token << ",";
        // }
        // std::cout << "]" << std::endl;
        resolving.push(tokens[0]);
        int i = 1;
        int finalResult = 0;
        while(resolving.size() > 0) {
            if (tokens[i] != ")") {
                resolving.push(tokens[i]);
            } else {
                vector<string> operation;
                while (resolving.top() != "(") {
                    operation.push_back(resolving.top());
                    resolving.pop();
                }
                // std::reverse(operation.begin(), operation.end());
                resolving.pop();
                resolving.push(evaluate(operation));
            }
            i++;
            if (i == tokens.size()) {
                vector<string> operation;
                while (resolving.size() > 0) {
                    operation.push_back(resolving.top());
                    resolving.pop();
                }
                // std::reverse(operation.begin(), operation.end());
                finalResult = std::stoi(evaluate(operation));
            }
        }
        return finalResult;
    }
};
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