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0227-basic-calculator-ii/0227-basic-calculator-ii.py

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+class Solution:
+    def calculate(self, s: str) -> int:
+        """
+        Complexities:
+        Time:  O(n)
+        Space: O(n)
+
+        where n = len(s)
+        """
+
+
+        numbers = "0123456789"
+
+        # Read from left to right: save in stack (full number or operator)
+        # - save number
+        # - on operator:
+        # -- if * or /, evaluate
+        # -- if + or -, wait and continue
+        currentNumber = 0
+        stack = []
+
+        # Evaluate numbers and create stack (of additions + subtractions)
+        for c in s + "\0":
+            if c == " ":
+                continue
+            elif c in numbers:
+                digit = int(c)
+                currentNumber = (currentNumber * 10) + digit
+            else:
+                stack.append(currentNumber)
+                currentNumber = 0
+
+                if len(stack) >= 3 and stack[-2] in "*/":
+                    rOperand = stack.pop()
+                    operator = stack.pop()
+                    lOperand = stack.pop()
+
+                    if operator == "*":
+                        stack.append(lOperand * rOperand)
+                    elif operator == "/":
+                        stack.append(int(lOperand / rOperand))
+
+                stack.append(c)
+
+        # Pop off null terminator
+        stack.pop()
+
+        # Evaluate all additions and subtractions from stack
+        current = stack[0]
+        for i in range(1, len(stack) - 1, 2):
+            operator = stack[i]
+            rOperand = stack[i + 1]
+
+            if operator == "+":
+                current += rOperand
+            elif operator == "-":
+                current -= rOperand
+
+        return current
+            

0227-basic-calculator-ii/README.md

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+<h2><a href="https://leetcode.com/problems/basic-calculator-ii">227. Basic Calculator II</a></h2><h3>Medium</h3><hr><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>