<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:
    // order of operation + 3 * 2 2
    int calculate(string s) 
    {
        string current_number ="";
        queue<string> operations;

        for(int i =0; i < s.length() ; i++)
        {
            if(s[i]==' ')
                continue;
            if(s[i] >= '0' && s[i] <= '9')
            {
                current_number.push_back(s[i]);
            }
            else if(s[i]== '*' || s[i] == '/')
            {
                string next_number = "";
                int count=i+1;
                while((s[count] >='0' && s[count]<='9' )|| s[count]==' ')
                {
                    if(s[count] != ' ')
                        next_number.push_back(s[count]);
                    count++;
                }

                string number;
                if(s[i] =='*')
                    number = to_string(stoi(current_number) * stoi(next_number));
                else
                    number = to_string(stoi(current_number) / stoi(next_number));

                i = count-1;
                current_number = number;
                next_number="";
            }
            else if(s[i] == '+'|| s[i] == '-' )
            {
                operations.push(current_number);
                if(s[i]=='+')
                    operations.push("+");
                else
                    operations.push("-");
                current_number="";
            }
        }

        if(current_number!="")
            operations.push(current_number);

        int result = stoll(operations.front());
        operations.pop();
        while(!operations.empty())
        {
            string oper = operations.front();
            operations.pop();

            int second = stoll(operations.front());
            operations.pop();

            if(oper=="+")
                result+= second;
            else
                result-= second;
        }
        return result;
    }
};
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