def test_peek():
s = Stack()
s.push("apple")
s.push("banana")
actual = s.peek()
expected = "banana"
assert actual == expected
def test_peek_empty():
s = Stack()
with pytest.raises(InvalidOperationError) as e:
s.peek()
assert str(e.value) == "Method not allowed on empty collection"
class StackMin2(Stack):
"""More efficient version of StackMin
Doesn't store extra piece of data (min value) at every node, since this is
v. inefficient if, say, first node is min value, and there are many nodes
above it.
Instead, use another stack to keep track of min value when it changes. Note
that adding another element with value equal to the current min value does
add to this stack, since otherwise removing this element would break
get_min() (this wasn't clear in book's solution).
^Above solution based on book's solution.
"""
def __init__(self):
"""Initialise stack as is done for Stack, but also introduce stack
to track min values
"""
# Initialise Stack
super().__init__()
# Use this stack to track min vals
self.min_vals = Stack()
def get_min(self):
"""Returns min value in stack
Returns
-------
min_val : any
Minimum value of sub-stack. Can be any Python object that allows
comparison.
"""
return self.min_vals.peek()
def push(self, data):
"""Add item (data) to top of stack (i.e. to head of Stack).
Ensure to add info about min of sub-array
Parameters
----------
data
Data to store in stack.
"""
# Find exising min_val
# Handle special case of empty stack
if self.head is None:
self.min_vals.push(data)
else:
min_val = self.get_min()
if data <= min_val:
self.min_vals.push(data)
# Add data to stack
self.prepend(data)
def pop(self):
"""Remove and return the top item from the stack.
Returns
-------
any
Data at head of Stack. Can be any Python object that allows
comparison.
"""
if self.head is None:
raise Exception('{} is empty.'.format(self.__name__))
# Retrieve head node data, then delete head node
data = self.head.data
self._delete_head()
# Remove element from min_vals stack, if required
if data == self.min_vals.peek():
self.min_vals.pop()
return data