在python中旋转列表最有效的方法是什么? 现在我有这样的东西:

>>> def rotate(l, n):
...     return l[n:] + l[:n]
... 
>>> l = [1,2,3,4]
>>> rotate(l,1)
[2, 3, 4, 1]
>>> rotate(l,2)
[3, 4, 1, 2]
>>> rotate(l,0)
[1, 2, 3, 4]
>>> rotate(l,-1)
[4, 1, 2, 3]

有没有更好的办法?


当前回答

我不知道这是否“有效”,但它也有效:

x = [1,2,3,4]
x.insert(0,x.pop())

编辑:再次你好,我刚刚发现这个解决方案的一个大问题! 考虑下面的代码:

class MyClass():
    def __init__(self):
        self.classlist = []

    def shift_classlist(self): # right-shift-operation
        self.classlist.insert(0, self.classlist.pop())

if __name__ == '__main__':
    otherlist = [1,2,3]
    x = MyClass()

    # this is where kind of a magic link is created...
    x.classlist = otherlist

    for ii in xrange(2): # just to do it 2 times
        print '\n\n\nbefore shift:'
        print '     x.classlist =', x.classlist
        print '     otherlist =', otherlist
        x.shift_classlist() 
        print 'after shift:'
        print '     x.classlist =', x.classlist
        print '     otherlist =', otherlist, '<-- SHOULD NOT HAVE BIN CHANGED!'

shift_classlist()方法执行的代码与我的x.insert(0,x.pop())-solution相同,otherlist是一个独立于类的列表。在将otherlist的内容传递给MyClass之后。Classlist列表,调用shift_classlist()也会改变otherlist列表:

控制台输出:

before shift:
     x.classlist = [1, 2, 3]
     otherlist = [1, 2, 3]
after shift:
     x.classlist = [3, 1, 2]
     otherlist = [3, 1, 2] <-- SHOULD NOT HAVE BIN CHANGED!



before shift:
     x.classlist = [3, 1, 2]
     otherlist = [3, 1, 2]
after shift:
     x.classlist = [2, 3, 1]
     otherlist = [2, 3, 1] <-- SHOULD NOT HAVE BIN CHANGED!

我使用Python 2.7。我不知道这是不是一个错误,但我认为更有可能是我误解了这里的一些东西。

有人知道为什么会这样吗?

其他回答

deque对两端的拉和推进行了优化。它们甚至有一个专用的rotate()方法。

from collections import deque
items = deque([1, 2])
items.append(3)        # deque == [1, 2, 3]
items.rotate(1)        # The deque is now: [3, 1, 2]
items.rotate(-1)       # Returns deque to original state: [1, 2, 3]
item = items.popleft() # deque == [2, 3]

以下函数将发送的列表复制到templist,这样pop函数不会影响原始列表:

def shift(lst, n, toreverse=False):
    templist = []
    for i in lst: templist.append(i)
    if toreverse:
        for i in range(n):  templist = [templist.pop()]+templist
    else:
        for i in range(n):  templist = templist+[templist.pop(0)]
    return templist

测试:

lst = [1,2,3,4,5]
print("lst=", lst)
print("shift by 1:", shift(lst,1))
print("lst=", lst)
print("shift by 7:", shift(lst,7))
print("lst=", lst)
print("shift by 1 reverse:", shift(lst,1, True))
print("lst=", lst)
print("shift by 7 reverse:", shift(lst,7, True))
print("lst=", lst)

输出:

lst= [1, 2, 3, 4, 5]
shift by 1: [2, 3, 4, 5, 1]
lst= [1, 2, 3, 4, 5]
shift by 7: [3, 4, 5, 1, 2]
lst= [1, 2, 3, 4, 5]
shift by 1 reverse: [5, 1, 2, 3, 4]
lst= [1, 2, 3, 4, 5]
shift by 7 reverse: [4, 5, 1, 2, 3]
lst= [1, 2, 3, 4, 5]

下面是一个高效的算法,不需要使用任何额外的数据结构:

旋转(nums: List[int], k: int):

    k = k%len(nums)
    l, r = 0, len(nums)-1
    while (l<r):
        nums[l], nums[r]= nums[r], nums[l]
        l,r=l+1,r-1
    
    l,r = 0, k-1
    while (l<r):
        nums[l], nums[r]=nums[r], nums[l]
        l,r=l+1,r-1
        
    l,r=k,len(nums)-1
    while (l<r):
        nums[l], nums[r]=nums[r], nums[l]
        l,r=l+1,r-1

如果效率是你的目标,(周期?内存?),您最好查看数组模块:http://docs.python.org/library/array.html

数组没有列表的开销。

就纯粹的列表而言,你所拥有的就是你所希望做的。

我是“老派”,我定义了最低延迟,处理器时间和内存使用效率,我们的克星是臃肿的库。所以只有一个正确的方法:

    def rotatel(nums):
        back = nums.pop(0)
        nums.append(back)
        return nums