How to Reverse a list using recursion in Python

In this post, you will learn how to reverse a list using recursion in python with a detailed explanation and algorithm.

To reverse a list using recursion we use a two-pointer approach. In this approach, we take two pointers L & R initially L point to the first element of the list and R points to the last element of the list.

Let’s understand whole concepts with the help of an example.

Explanation

Take one list L1 = [3,1,5,4,7] where initially L points to the first element of the list and R points to the last element of the list as shown below.

First swap the element at L with the element at R after that increment L (L += 1) & decrement R (R -= 1).

repeat the above step ( first swap the element at L with the element at R after that increment L & decrement R).

if L >= R that means we have completed the task to reverse a list.

Here in this case L == R means the list is reversed.

reverse of the list l1 = [7,4,5,1,3].

Algorithm to Reverse a list using recursion

• Take two-pointers L & R.
• L point to the first element of the list and R point to the last element of the list.
• if L >= R then returns your list (return my_list).
• otherwise, swap my_list[L] with my_list[R]
• At the end return recursive function reverse_list(my_list,L+1,R-1).

Now we have understood how to write a program to reverse a list using recursion through the above algorithm.

But before we write a program few concepts you have to know:

1. if-else
2. function in python &
3. How recursive function work

Source code

def reverse_list(my_list, l, r):
    if l >= r:
        return my_list

    #swap
    tem = my_list[l]
    my_list[l] = my_list[r]
    my_list[r] = tem

    return reverse_list(my_list,l+1,r-1)


my_l = [3,1,5,4,7]

L = 0
R = len(my_l)-1

print('my list:',my_l)

print('reverse list :',reverse_list(my_l,L,R))

Output

my list: [3, 1, 5, 4, 7]
reverse list : [7, 4, 5, 1, 3]

Fun Fact: In python, it is very simple to reverse a list, you just use slicing and step argument to reverse a list (list_name[::-1])

But for recursion, these are general steps you have to follow.