Python的deque適用於需要高效處理兩端操作的場景。 1. 創(chuàng)建時可傳入列表或字符串，也可初始化為空再添加元素；2. 使用append()和appendleft()分別在右端和左端添加元素；3. 使用pop()和popleft()分別從右端和左端移除元素；4. rotate(n)方法將元素向右（正數(shù)）或向左（負數(shù)）循環(huán)移動；5. 設置maxlen參數(shù)後，超出容量時會自動丟棄對端舊元素；6. 適合用於隊列、滑動窗口、歷史記錄等場景。相比列表，deque在首部操作具有O(1)時間複雜度，性能更優(yōu)。

The deque (double-ended queue) from Python's collections module is a versatile and efficient data structure, especially when you need fast appends and pops from both ends. If you're used to working with lists, switching to deque can give you performance boosts in certain situations.

Let's break down how to use it effectively.

Creating and Initializing a Deque

To start using deque , you first need to import it from the collections module. Then you can create one by passing in an iterable like a list or string.

 from collections import deque

d = deque([1, 2, 3])

You can also initialize it empty and add elements later. It's pretty flexible — strings, tuples, and even other deques work as input.

If you're starting from scratch:

• Use append() to add to the right end
• Use appendleft() to add to the left end

 d = deque()
d.append(1) # deque([1])
d.appendleft(0) # deque([0, 1])

Adding and Removing Elements Efficiently

One of the main advantages of deque over regular lists is its speed for operations at both ends. With a normal list, inserting or removing from the front ( pop(0) or insert(0, x) ) takes O(n) time, which gets slow for large data sets. deque does these operations in O(1) time.

Here are some common operations:

• Add to the right : append(x)
• Add to the left : appendleft(x)
• Remove from the right : pop()
• Remove from the left : popleft()

 d = deque([1, 2, 3])
d.append(4) # deque([1, 2, 3, 4])
d.popleft() # returns 1 → deque([2, 3, 4])

This makes deque perfect for things like queues or sliding window problems.

Rotating and Managing Elements

Another handy feature is the rotate() method. It shifts elements in place to the right (or left if given a negative number).

For example:

 d = deque([1, 2, 3, 4, 5])
d.rotate(1) # deque([5, 1, 2, 3, 4])

That moves each element one position to the right, wrapping around the end. A negative rotation goes the other way:

 d.rotate(-1) # back to deque([1, 2, 3, 4, 5])

Also, if you ever need to limit the size of your deque, you can set the maxlen parameter when creating it. Once full, adding new items will automatically drop the oldest ones from the opposite end.

 d = deque(maxlen=3)
d.append(1)
d.append(2)
d.append(3)
d.append(4) # now contains [2, 3, 4]

This is super useful for tracking recent values or implementing fixed-size buffers.

When to Use Deque Instead of List

In most cases, you'll still want to use a regular list. But if you find yourself frequently doing:

• Insertions/removals at the beginning
• Implementing queues or stacks
• Maintaining a history or buffer of recent items

Then deque is the better choice.

Even though they look similar and support many of the same methods, their performance characteristics differ. So if you're building something that needs high efficiency on both ends, switch to deque .

基本上就這些。掌握幾個常用方法之後，你會發(fā)現(xiàn)它在很多場景下比列表更合適，而且用起來也不復雜。

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