Given a text file. The task is to reverse a single line of user’s choice from a given text file and update the already existing file. Examples:
Input:
Hello Geeks
for Lazyroar!
User choice = 1
Output:
Hello Geeks
Lazyroar! for
Input:
This is a geek
Welcome to Lazyroar
Lazyroar is a computer science portal
User choice = 0
Output:
geek a is This
Welcome to Lazyroar
Lazyroar is a computer science portal
Implementation: Let’s suppose the text file looks like this – 
Python3
# Open file in read modef = open('GFG.txt', 'r')# Read the content of the# file and store it in a listlines = f.readlines() # Close filef.close()# User's choicechoice = 1# Split the line into words line = lines[choice].split()# line is reversedReversed = " ".join(line[::-1])# Updating the content of the# filelines.pop(choice)lines.insert(choice, Reversed)# Open file in write modeu = open('GFG.txt', 'w')# Write the new content in file# and note, it is overwritten u.writelines(lines)u.close() |
Output: 
Time complexity: O(n), where n is the number of lines in the file.
Auxiliary space: O(n), where n is the number of lines in the file.
