# basic-calculator-ii


Try it on <a href='https://leetcode.com/problems/basic-calculator-ii'>leetcode</a>

## Description
<div class="description">
<div><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>Example 1:</strong></p>
<pre><strong>Input:</strong> s = "3+2*2"
<strong>Output:</strong> 7
</pre><p><strong>Example 2:</strong></p>
<pre><strong>Input:</strong> s = " 3/2 "
<strong>Output:</strong> 1
</pre><p><strong>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>('+', '-', '*', '/')</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>
</div>
</div>

## Solution(Python)
```Python
class Solution:
    def calculate(self, s: str) -> int:
        inner, outer, result, opt = 0, 0, 0, "+"
        for c in s + "+":
            if c == " ":
                continue
            if c.isdigit():
                inner = 10 * inner + int(c)
                continue
            if opt == "+":
                result += outer
                outer = inner
            elif opt == "-":
                result += outer
                outer = -inner
            elif opt == "*":
                outer = outer * inner
            elif opt == "/":
                outer = int(outer / inner)
            inner, opt = 0, c
        return result + outer

```