Miguel 3.2 - Stack Min [Python] (#83)

Author: miguelHx

Python/chapter03/p02_stack_min/miguelHx.py

@@ -0,0 +1,248 @@
+"""
+Python version 3.7.0
+3.2 - Stack Min
+How would you design a stack which, in addition to
+push and pop, has a function min which returns the minimum element?
+Push, pop and min should all operate in O(1) time.
+"""
+
+import copy
+import unittest
+import sys
+
+from abc import abstractmethod
+from dataclasses import dataclass
+from typing import Generic, TypeVar
+from typing import List, Optional, Iterator
+
+from typing import Protocol
+
+T = TypeVar('T', bound='Comparable')
+
+class Comparable(Protocol):
+    @abstractmethod
+    def __lt__(self, other: T) -> bool:
+        pass
+
+@dataclass
+class StackNode(Generic[T]):
+    data: T
+    next: 'Optional[StackNode[T]]'
+    running_min: T
+
+class MyStack(Generic[T]):
+    """Stack data structure implementation.
+    Uses LIFO (last-in first-out) ordering.
+    The most recent item added to the stack is
+    the first removed.  Traversal is top to bottom.
+    """
+
+    class MyStackIterator(Generic[T]):
+        def __init__(self, top: Optional[StackNode[T]], size: int):
+            self.index = -1
+            self.current_node = top
+            self._size = size
+
+        def __next__(self) -> T:
+            self.index += 1
+            if self.index == self._size or self.current_node is None:
+                raise StopIteration
+            n: T = self.current_node.data
+            self.current_node = self.current_node.next
+            return n
+
+    def __init__(self):
+        self.top: Optional[StackNode[T]] = None # top is a pointer to StackNode object
+        self._size: int = 0
+    
+    def pop(self) -> T:
+        """
+        Removes the top item from the stack
+
+        Raises:
+            IndexError: raised when pop is attempted on empty stack
+
+        Returns:
+            int: The data at the top of the stack
+        """
+        if self.top is None:
+            raise IndexError('Stack is Empty.')
+        item = self.top.data
+        self.top = self.top.next
+        self._size -= 1
+        return item
+    
+    def push(self, item: T) -> None:
+        """
+        Adds an item to the top of the stack
+        Args:
+            item (T): data we want at the top of stack
+        """
+        t: StackNode = StackNode(item, None, sys.maxsize)
+        t.next = self.top
+        if self.top is None:
+            t.running_min = item
+        elif item < self.top.data: # we will assume data implements __lt__
+            t.running_min = item
+        else:
+            t.running_min = self.top.running_min
+        # print("Stack Node data = {}, local min = {}".format(item, t.running_min))
+        self.top = t
+        self._size += 1
+
+    def min(self) -> T:
+        """
+        Returns the element with the lowest value in the stack
+
+        Returns:
+            int: min value in stack
+        """
+        if self.top is None:
+            raise ValueError('Min does not exist yet because there are no values in the stack.')
+        return self.top.running_min
+
+    def peek(self) -> T:
+        """
+        Returns data at the top of the stack
+
+        Raises:
+            IndexError: [description]
+
+        Returns:
+            int: the value at the top of the stack
+        """
+        if self.top is None:
+            raise IndexError('Stack is Empty')
+        return self.top.data
+    
+    def __iter__(self) -> Iterator:
+        """
+        Builds a list of the current stack state.
+        For example, given the following stack:
+            3 -> 2 -> 1, where 3 is the top,
+        Expect:
+            [3, 2, 1]
+        Returns:
+            List[int]: list of integers
+        """
+        return self.MyStackIterator(self.top, self._size)
+    
+    def __bool__(self) -> bool:
+        """
+        True is returned when the container is not empty.
+        From https://docs.python.org/3/reference/datamodel.html#object.__bool__ :
+        Called to implement truth value testing and the built-in operation bool();
+        should return False or True. When this method is not defined, len() is called,
+        if it is defined, and the object is considered true if its result is nonzero.
+        If a class defines neither len() nor bool(), all its instances are considered true.
+        Returns:
+            bool: False when empty, True otherwise
+        """
+        return self._size > 0
+    
+    def __len__(self) -> int:
+        return self._size
+    
+    def __str__(self):
+        if self._size == 0:
+            return '<Empty>'
+        values = []
+        n = self.top
+        while n.next is not None:
+            values.append(str(n.data))
+            n = n.next
+        values.append(str(n.data))
+        return '->'.join(values)      
+
+
+class TestMyStack(unittest.TestCase):
+    def test_stack_push(self):
+        s = MyStack()
+        self.assertEqual(len(s), 0)
+        self.assertEqual(s.top, None)
+        s.push(2)
+        self.assertEqual(len(s), 1)
+        self.assertEqual(s.top.data, 2)
+        self.assertEqual(s.top.next, None)
+        s.push(3)
+        self.assertEqual(len(s), 2)
+        self.assertEqual(s.top.data, 3)
+        self.assertEqual(s.top.next.data, 2)
+        s.push(4)
+        self.assertEqual(len(s), 3)
+        self.assertEqual(s.top.data, 4)
+        self.assertEqual(s.top.next.data, 3)
+        self.assertEqual(list(s), [4, 3, 2])
+
+        # test adding different types, (float and int)
+        s.push(1.2)
+        self.assertEqual(len(s), 4)
+        self.assertEqual(s.top.data, 1.2)
+        self.assertEqual(s.top.next.data, 4)
+        self.assertEqual(list(s), [1.2, 4, 3, 2])
+
+    def test_stack_peek(self):
+        s = MyStack()
+        with self.assertRaises(IndexError):
+            s.peek()
+        s.push(1)
+        s.push(2)
+        s.push(99)
+        top_val = s.peek()
+        self.assertEqual(top_val, 99)
+    
+    def test_stack_pop(self):
+        # first case, attempt to pop an empty stack
+        s = MyStack()
+        with self.assertRaises(IndexError):
+            s.pop()
+        s.push(1)
+        s.push(2)
+        s.push(3)
+        # size is 3
+        self.assertEqual(list(s), [3, 2, 1])
+        val = s.pop()
+        self.assertEqual(val, 3)
+        self.assertEqual(s._size, 2) # size should now be 2
+        self.assertEqual(list(s), [2, 1])
+    
+    def test_stack_min(self):
+        s = MyStack()
+        s.push(9)
+        self.assertEqual(s.min(), 9)
+        s.push(7)
+        self.assertEqual(s.min(), 7)
+        s.push(-2)
+        self.assertEqual(s.min(), -2)
+        s.push(5)
+        self.assertEqual(s.min(), -2)
+        s.push(8)
+        self.assertEqual(s.min(), -2)
+        s.push(-99)
+        self.assertEqual(s.min(), -99)
+        s.push(2)
+        self.assertEqual(s.min(), -99)
+        s.pop()
+        self.assertEqual(s.min(), -99)
+        s.pop()
+        self.assertEqual(s.min(), -2)
+        s.pop()
+        self.assertEqual(s.min(), -2)
+        s.pop()
+        self.assertEqual(s.min(), -2)
+        s.pop()
+        self.assertEqual(s.min(), 7)
+        s.pop()
+        self.assertEqual(s.min(), 9)
+        s.pop()
+        self.assertTrue(not s)
+    
+    def test__bool__(self):
+        s = MyStack()
+        self.assertFalse(s)
+        s.push(3)
+        self.assertTrue(s)
+
+
+if __name__ == '__main__':
+    unittest.main()