<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:
    deque<int> qn;
    queue<char> qs;
    int calculate(string s) {
        string tmptmp = "";
        for (int i = 0; i < s.size(); i++)
        {
            if (s[i] != ' ') tmptmp += s[i];
        }
        s = tmptmp;
        for (int i = 0; i < s.size(); i++)
        {
            if ('0' <= s[i] && s[i] <= '9')
            {
                string tmps = "";
                while (i < s.size() && '0' <= s[i] && s[i] <= '9')
                {
                    tmps += s[i];
                    i++;
                }
                i--;
                qn.push_back(stoi(tmps));
            }
            else
            {
                if (s[i] == '+' || s[i] == '-')
                {
                    qs.push(s[i]);
                }
                else
                {
                    char tmp_sign = s[i];
                    int prev = qn.back();
                    qn.pop_back();
                    string tmps = "";
                    i++;
                    while (i < s.size() && '0' <= s[i] && s[i] <= '9')
                    {
                        tmps += s[i];
                        i++;
                    };
                    i--;
                    int post = stoi(tmps);

                    if (tmp_sign == '*')
                    {
                        qn.push_back(post * prev);
                    }
                    else if (tmp_sign == '/')
                    {
                        qn.push_back(prev / post);
                    }
                }
            }
        }

        while (!qn.empty())
        {
            int n1 = qn.front();
            if (qs.empty())
            {
                return n1;
            }
            qn.pop_front();
            int n2 = qn.front();
            qn.pop_front();
            char c = qs.front();
            qs.pop();
            if (c == '+')
            {
                qn.push_front(n1 + n2);
            }
            else if (c == '-')
            {
                qn.push_front(n1 - n2);
            }
        }

        return -1;
    }
};
18681