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# Stacks (LIFO) / Queues (FIFO)

With a queue, you remove the item least recently added (first-in, first-out or FIFO); but with a stack, you remove the item most recently added (last-in, first-out or LIFO).

## Stacks (LIFO)

### \`collections.deque\` – Fast & Robust Stacks

```python
>>> from collections import deque
>>> s = deque()
>>> s.append('eat')
>>> s.append('sleep')
>>> s.append('code')

>>> s
deque(['eat', 'sleep', 'code'])

>>> s.pop()
'code'
>>> s.pop()
'sleep'
>>> s.pop()
'eat'

>>> s.pop()
IndexError: "pop from an empty deque"
```

## Queues (FIFO)

```python
>>> from collections import deque
>>> q = deque()
>>> q.append('eat')
>>> q.append('sleep')
>>> q.append('code')

>>> q
deque(['eat', 'sleep', 'code'])

>>> q.popleft()
'eat'
>>> q.popleft()
'sleep'
>>> q.popleft()
'code'

>>> q.popleft()
IndexError: "pop from an empty deque"
```

**`list`** is backed by a dynamic array which makes it great for fast random access, but requires occasional resizing when elements are added or removed. The list over-allocates its backing storage so that not every push or pop requires resizing, and you get an amortized O(1) time complexity for these operations. But you do need to be careful to only insert and remove items “from the right side” using `append()` and `pop()`. Otherwise, performance slows down to O(n). 

**`collections.deque`** is backed by a doubly-linked list which optimizes appends and deletes at both ends and provides consistent O(1) performance for these operations. Not only is its performance more stable, the `deque` class is also easier to use because you don’t have to worry about adding or removing items from “the wrong end.”


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