class MyQueue: def __init__(self): self.queue = Stack() def add(self, item): self.queue.push(item) def remove(self): temp = self.queue.reverse() res = temp.pop() self.queue = temp.reverse() return res def peek(self): temp = self.queue.reverse() res = temp.peek() self.queue = temp.reverse() return res def is_empty(self): return self.queue.is_empty() def __str__(self): return str(self.queue)
def get_path(graph, from_s, from_t):
"""Traces the searchers to create and return a list of indices
representing the shortest path from s to t. Assumes that the
search is already complete and that a path has been found.
Args:
graph: A Graph instance.
from_s, from_t: Searcher instances.
Returns:
A list of integers.
"""
path = []
# We first construct the path from s to the connection point. Using
# a stack ensures that the order is correct.
start_stack = Stack()
while from_s.backtracer is not None:
start_stack.push(from_s.backtracer)
from_s.backtracer = graph[from_s.backtracer].backpointer
while not start_stack.is_empty():
path.append(start_stack.pop())
# Then we extend the path from the connection point to t.
while from_t.backtracer is not None:
path.append(from_t.backtracer)
from_t.backtracer = graph[from_t.backtracer].backpointer
return path
class SetOfStacks:
max_items = 3
def __init__(self):
self.piles = Stack()
self._create_new_stack()
def _create_new_stack(self):
self.piles.push(Stack())
def push(self, item):
if self.piles.peek().size() == self.max_items:
self._create_new_stack()
self.push(item)
else:
self.piles.peek().push(item)
def pop(self):
return self.piles.peek().pop()
def pop_stack(self):
return Stack.pop(self.piles)
def is_empty(self):
return self.piles.is_empty()
def _backfill(self, new, old):
while new.size() > 0:
old.push(new.pop())
def pop_at(self, index):
if index > self.piles.size()-1:
raise "Invalid index."
temp_stack = Stack()
for i in range(index, 0, -1):
temp_stack.push(SetOfStacks.pop_stack(self))
item = self.piles.pop()
self._backfill(temp_stack.reverse(), self.piles)
return item.pop()
def __str__(self):
if not self.is_empty():
return Stack.__str__(self.piles)
return "Stack is empty."
def test_stack_with_no_elements_is_empty():
s = Stack()
assert s.is_empty()
