<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) {
        /*
            if its a mult or divide, go to next num and then do it, if its an add just put it onto stack
            
        */
        s += "-";
        long long currentNum = 0;
        char lastOp = '0';
        vector<long long> st;
        for (char c: s) {
            if (c == ' ') { continue; }
            if (c <= '9' && c >= '0') {
                currentNum *= 10;
                currentNum += (lastOp == '-' ? '0' - c : c - '0' );
                // if (lastOp == '-') {
                //     currentNum = min(-1*currentNum, currentNum);
                // }
                continue;
            }
            if (lastOp == '*' || lastOp == '/') {
                long long last = st.back();
                if (lastOp == '*')
                    st[st.size() - 1] = last * currentNum;
                else
                    st[st.size() - 1] = last / currentNum;
            } else {
                st.push_back(currentNum);
            }
            lastOp = c;
            currentNum = 0;
        }
        return accumulate(st.begin(), st.end(), 0);

    }
};
17854