Python – Replacing by Greatest Neighbour in list

Given a list, the task is to write a Python program to replace with the greatest neighbor among previous and next elements.

Input : test_list = [5, 4, 2, 5, 8, 2, 1, 9], 
Output : [5, 5, 5, 8, 8, 8, 9, 9] 
Explanation : 4 is having 5 and 2 as neighbours, replaced by 5 as greater than 2.

Input : test_list = [5, 4, 2, 5], 
Output : [5, 5, 5, 5] 
Explanation : 4 is having 5 and 2 as neighbours, replaced by 5 as greater than 2. 

Method 1 : Using loop + chain conditional statements

In this, we use loop to iterate through all the elements in list and check for neighbours for greater element using conditionals and then is replaced.

Python3

# Python3 code to demonstrate working of
# Replacing by Greatest Neighbour
# Using loop + chain conditional statements

# initializing list
test_list = [5, 4, 2, 5, 8, 2, 1, 9]

# printing original list
print("The original list is : " + str(test_list))

for idx in range(1, len(test_list) - 1):

    # replacing by greater Neighbour
    test_list[idx] = test_list[idx - 1] \
    if test_list[idx - 1] > test_list[idx + 1] \
    else test_list[idx + 1]

# printing result
print("The elements after replacing : " + str(test_list))

Output:

The original list is : [5, 4, 2, 5, 8, 2, 1, 9]
The elements after replacing : [5, 5, 5, 8, 8, 8, 9, 9]

Time Complexity: O(n)
Auxiliary Space: O(1)

Method 2 : Using max() + loop.

In this, we get the maximum element among neighbouring elements using max(). The loop is used to iterate through the elements.

Python3

# Python3 code to demonstrate working of
# Replacing by Greatest Neighbour
# Using max() + loop

# initializing list
test_list = [5, 4, 2, 5, 8, 2, 1, 9]

# printing original list
print("The original list is : " + str(test_list))

for idx in range(1, len(test_list) - 1):

    # using max() to get maximum of Neighbours
    test_list[idx] = max(test_list[idx - 1], test_list[idx + 1])

# printing result
print("The elements after replacing : " + str(test_list))

Output:

The original list is : [5, 4, 2, 5, 8, 2, 1, 9]
The elements after replacing : [5, 5, 5, 8, 8, 8, 9, 9]

Time Complexity: O(n*n)
Auxiliary Space: O(n)