<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>

class Solution {
public:
    int calculate(string s) {
        int ans = 0;
        string num = "";
        char op;

        int pos1 = 0;
        int i = 0;
        while (i < s.length()) {
            char c = s[i];
            if (c == ' ') {i++; continue;}
            if (c == '+' || c == '-') { pos1 = i+1; i++;}
            else if (c == '*' || c == '/') {
                while (s[pos1] > '9' || s[pos1] < '0') pos1++;
                string num1 = s.substr(pos1, i - pos1);
                int j = i + 1;
                while (s[j] > '9' || s[j] < '0') j++;
                int pos2 = j;
                while (s[j] <= '9' && s[j] >= '0') j++;
                string num2 = s.substr(pos2, j - pos2);
                int res = 0;
                if (c == '*') res = stoi(num1) * stoi(num2);
                else res = stoi(num1) / stoi(num2);
                s = s.substr(0, pos1) + to_string(res) + s.substr(j, s.length() - j);
                i = pos1 + to_string(res).length();
            }
            else i++;
        }

        op = 'b';
        for (int i = 0; i < s.length(); i++) {
            char c = s[i];
            if (c == ' ') continue;
            if (c >= '0' && c <= '9') num += c;
            else {
                if (num == "") {
                    op = c;
                    continue;
                }
                if (op == '+' || op == 'b') ans += stoi(num);
                else if (op == '-') ans -= stoi(num);
                // else if (op == '*') ans *= stoi(num);
                // else if (op == '/') ans /= stoi(num);
                op = c;
                num = "";
            }
        }
        if (op == '+' || op == 'b') ans += stoi(num);
        else if (op == '-') ans -= stoi(num);
        return ans;
    }
};
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