Python program to right rotate a list by n

Given a list and an integer n, write a Python program to right rotate the list by n position.

Examples :

Input : n = 2, List_1 = [1, 2, 3, 4, 5, 6]
Output : List_1 = [5, 6, 1, 2, 3, 4]
Explanation: We get output list after right rotating (clockwise) given list by 2.

Input :  n = 3, List_1 = [3, 0, 1, 4, 2, 3]
Output : List_1 = [4, 2, 3, 3, 0, 1]

Approach #1 : Traverse the first list one by one and then put the elements at required places in a second list. 

Python3

# Python program to right rotate a list by n

# Returns the rotated list

def rightRotate(lists, num):
    output_list = []

    # Will add values from n to the new list
    for item in range(len(lists) - num, len(lists)):
        output_list.append(lists[item])

    # Will add the values before
    # n to the end of new list
    for item in range(0, len(lists) - num):
        output_list.append(lists[item])

    return output_list


# Driver Code
rotate_num = 3
list_1 = [1, 2, 3, 4, 5, 6]

print(rightRotate(list_1, rotate_num))

Output

[4, 5, 6, 1, 2, 3]

Time complexity : O(n)

Approach #2 : Another approach to solve this problem by using slicing technique. One way of slicing list is by using len() method. 

Python3

# Python program to right rotate
# a list by n using list slicing
n = 3

list_1 = [1, 2, 3, 4, 5, 6]
list_1 = (list_1[len(list_1) - n:len(list_1)]
                 + list_1[0:len(list_1) - n])
print(list_1)

Output

[4, 5, 6, 1, 2, 3]

Approach #3 : In the above method, last n elements of list_1 was taken and then remaining elements of list_1. Another way is without using len() method.  

Python

# Right Rotating a list to n positions
n = 3

list_1 = [1, 2, 3, 4, 5, 6]
if n>len(list_1):
    n = int(n%len(list_1))
list_1 = (list_1[-n:] + list_1[:-n])

print(list_1)

Output

[4, 5, 6, 1, 2, 3]

Time complexity : O(n)

Note : list_1[:] will return the whole list as the blank space on left of slicing operator refers to start of list i.e 0 and blank space on right refers to ending position of list. 

Approach 4: One additional approach that could be used to right rotate a list by n positions is to use the collections.deque module. This module provides a doubly-linked list that supports fast insertion and deletion at both ends of the list, as well as fast rotating operations.

To right rotate a list by n positions, you can use the rotate() method of the deque object. This method rotates the elements of the list by the specified number of positions to the right. For example:

Python3

from collections import deque

# Create a deque object from the list
list_1 = [1, 2, 3, 4, 5, 6]
deque_1 = deque(list_1)

# Rotate the deque by 3 positions to the right
deque_1.rotate(3)

# Convert the deque back to a list
list_1 = list(deque_1)

print(list_1)  # Output: [4, 5, 6, 1, 2, 3]

Output

[4, 5, 6, 1, 2, 3]

The time complexity of the deque.rotate() method is O(n), because it involves shifting all the elements in the deque by a certain number of positions. The time complexity of the list() and deque() constructors is also O(n), because they involve iterating through all the elements in the input list or deque and creating a new list or deque with those elements.

The Auxiliary space complexity of the code is also O(n), because it involves creating a new deque and a new list, both of which have the same size as the input list.