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

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]

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]

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。

def solution(A, K):
    if len(A) == 0:
        return A

    K = K % len(A)

    return A[-K:] + A[:-K]

# use case
A = [1, 2, 3, 4, 5, 6]
K = 3
print(solution(A, K))

例如,给定

A = [3, 8, 9, 7, 6]
K = 3

函数应该返回[9,7,6,3,8]。进行了三次轮换:

[3, 8, 9, 7, 6] -> [6, 3, 8, 9, 7]
[6, 3, 8, 9, 7] -> [7, 6, 3, 8, 9]
[7, 6, 3, 8, 9] -> [9, 7, 6, 3, 8]

再举一个例子

A = [0, 0, 0]
K = 1

函数应该返回[0,0,0]

鉴于

A = [1, 2, 3, 4]
K = 4

函数应该返回[1,2,3,4]

对于一个列表X = ['a', 'b', 'c', 'd', 'e', 'f'],并且shift值小于列表长度,我们可以如下所示定义函数list_shift()

def list_shift(my_list, shift):
    assert shift < len(my_list)
    return my_list[shift:] + my_list[:shift]

的例子,

list_shift (X, 1)返回(' b ', ' c ', ' d ', ' e ', ' f ', ' '] list_shift (X, 3)返回(' d ', ' e ', ' f ', ' ', ' b ', ' c ']

我认为你有最有效的方法

def shift(l,n):
    n = n % len(l)  
    return l[-U:] + l[:-U]