<p>Given a string <code>s</code> which represents an expression, <em>evaluate this expression and return its value</em>.&nbsp;</p>

<p>The integer division should truncate toward zero.</p>

<p>You may assume that the given expression is always valid. All intermediate results will be in the range of <code>[-2<sup>31</sup>, 2<sup>31</sup> - 1]</code>.</p>

<p><strong>Note:</strong> You are not 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 = "3+2*2"
<strong>Output:</strong> 7
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> s = " 3/2 "
<strong>Output:</strong> 1
</pre><p><strong class="example">Example 3:</strong></p>
<pre><strong>Input:</strong> s = " 3+5 / 2 "
<strong>Output:</strong> 5
</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 integers and operators <code>(&#39;+&#39;, &#39;-&#39;, &#39;*&#39;, &#39;/&#39;)</code> separated by some number of spaces.</li>
	<li><code>s</code> represents <strong>a valid expression</strong>.</li>
	<li>All the integers in the expression are non-negative integers in the range <code>[0, 2<sup>31</sup> - 1]</code>.</li>
	<li>The answer is <strong>guaranteed</strong> to fit in a <strong>32-bit integer</strong>.</li>
</ul>

#include <iostream>
#include <stack>
#include <string>
#include <climits>

class Solution {
public:
    std::string number(const std::string& s, int ind) {
        std::string no = "";
        for (int i = ind + 1; i < s.size(); i++) {
            if (s[i] == '+' || s[i] == '-' || s[i] == '*' || s[i] == '/') {
                break;
            }
            no = no + s[i];
        }
        return no;
    }

    int calculate(std::string old) {
        std::string s = "";
        for (char c : old) {
            if (c != ' ') {
                s += c;
            }
        }

        std::stack<std::string> st;
        std::string no = "";
        int ind = -1;
        for (int i = 0; i < s.size(); i++) {
            if (s[i] == '+' || s[i] == '-' || s[i] == '*' || s[i] == '/') {
                ind = i;
                break;
            }
            no = no + s[i];
        }
        st.push(no);
        int i = ind;
        while (i < s.size()) {
            if (s[i] == '+' || s[i] == '-') {
                std::string a = number(s, i);
                st.push(s[i] + a);
                i = i + a.size() + 1;
            } else {
                std::string a = number(s, i);
                std::string b = st.top();
                st.pop();
                long long res;
                if (s[i] == '/') {
                    res = static_cast<long long>(std::stoi(b)) / std::stoi(a);
                } else {
                    res = static_cast<long long>(std::stoi(b)) * std::stoi(a);
                }
                res = std::max(static_cast<long long>(INT_MIN), std::min(res, static_cast<long long>(INT_MAX)));
                st.push(std::to_string(static_cast<int>(res)));
                i = i + a.size() + 1;
            }
        }
        long long sum = 0;
        while (!st.empty()) {
            sum += std::stoi(st.top());
            st.pop();
        }
        sum = std::max(static_cast<long long>(INT_MIN), std::min(sum, static_cast<long long>(INT_MAX)));
        return static_cast<int>(sum);
    }
};
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