Python中的deque（Doubly Ended Queue）是使用collections模块实现的。
在我们需要从容器的两端更快地执行append和pop操作的情况下，与列表相比，使用双端队列更可取，因为与O（n）时间复杂度的列表相比，双端队列为append和pop操作提供了O（1）时间复杂度。
- append() :- This function is used to insert the value in its argument to the right end of deque.
- appendleft() :- This function is used to insert the value in its argument to the left end of deque.
- pop() :- This function is used to delete an argument from the right end of deque.
- popleft() :- This function is used to delete an argument from the left end of deque.
- index(ele, beg, end) :- This function returns the first index of the value mentioned in arguments, starting searching from beg till end index.
- insert(i, a) :- This function inserts the value mentioned in arguments(a) at index(i) specified in arguments.
- remove() :- This function removes the first occurrence of value mentioned in arguments.
- count() :- This function counts the number of occurrences of value mentioned in arguments.
- extend(iterable) :- This function is used to add multiple values at the right end of deque. The argument passed is an iterable.
- extendleft(iterable) :- This function is used to add multiple values at the left end of deque. The argument passed is an iterable. Order is reversed as a result of left appends.
- reverse() :- This function is used to reverse order of deque elements.
- rotate() :- This function rotates the deque by the number specified in arguments. If the number specified is negative, rotation occurs to left. Else rotation is to right.
- in:- This function is to find the element is in the deque or not
- copy:- This function is to copy a deque
