[Solved] Reverse String Without Affecting Special Characters
Problem Statement: You have given a string. There can have multiple special characters inside the string. Write a program to reverse the given string. The position of the special characters should not be changed.
Example:
Input: 'abc/defgh$ij' Output: 'jih/gfedc$ba'
This question was asked in the Oracle interview.
Algorithm
Here is a simple algorithm to solve this problem to reverse string without affecting special characters
- Convert given string into a list (or array) such that every character in the string will be one element in the list.
- Set the two pointers at the beginning of the list (says i) and at the end of the list (says j)
- repeat while i<j:
- if list[i] is a special character, increment i by 1
- else if list[j] is a special character, decrement j by 1
- else (list[i] and list[j] are alphabets) swap list[i] and list[j], increment i by 1 , decrement j by 1
Python Program
strSample='abc/defgh$ij'
#convert string into list
listSample=list(strSample)
i=0
j=len(listSample)-1
while i<j:
if not listSample[i].isalpha():
i+=1
elif not listSample[j].isalpha():
j-=1
else:
#swap the element in the list
#if both elements are alphabets
listSample[i], listSample[j]=listSample[j], listSample[i]
i+=1
j-=1
#convert list into string
#by concatinating each element in the list
strOut=''.join(listSample)
print(strOut)Output:
jih/gfedc$ba
An isAlpha() is the character method that returns true if the character is an alphabet. Otherwise (if it is a special character), it returns false.
In Python, we can reverse the string with a single line of code. But, this is a special case where we don’t want to change the position of the special characters.
Similarly, you can solve this problem in C/C++ or Java.
Complexity
We are traversing each character in the string at once. In the worst case, the time complexity is O(n), where n is the size of the given string.
We can do the in-place swapping of the characters of the string. So, it does not require any extra space. (In the case of Python, it’s not possible to swap the characters inside the string. So, we are obligated to use the list. This causes extra memory space.)
Other Python Coding Questions for Practice:
This is the simple solution to reverse string without affecting special characters. Practice solving more of such coding questions to improve your programming skills.
Comments
abhishek
Hi,
How to do code for below one?
Sample Input:
Aniruddha Chaudhari
In this case, the words of the given string are reversed. Here is how you do this.
1. Split the words from the strings
2. Loop over all the words in the string
3. Reverse each word and concatenate it to the output string.
Let me know if it is not clear.
Milan
Hi Abishek, do you got the solution for that?
rahul
yes not cleared
def reverse_eachword(sentence): return " ".join([x[::-1] for x in sentence.split()]) print(reverse_eachword("String; 2be reversed..."))but o/p is not coming as expected
Aniruddha Chaudhari
Rahul, the Program you have written is to reverse each individual word in the sentence.
It returns.
Aashish
Hi Abhishek,
below is code which work for you:
str_smpl = 'String; 2be reversed...' lst = [] for word in str_smpl.split(' '): letters = [c for c in word if c.isalpha()] for c in word: if c.isalpha(): lst.append(letters.pop()) continue else: lst.append(c) lst.append(' ') print("".join(lst))Output:
Birobrata Deb
An additional space gets appended at the end of the reversed string: “gnirtS; 2eb desrever… “
Santosh
Hi,
How do I include assert in the main() condition which must succeed upon error?
Aniruddha Chaudhari
I did not get your comment. You can refer to Python assert statement.
Dwarakanath
str = list('abcdefghijklmnopqrstuvwxyz') user_input = input("enter the value -- ") print(user_input) list0 = list(user_input) list1 = list(user_input)[::-1] def reverse(user_input): for i in list1: if i not in str: list1.remove(i) list1.insert(list0.index(i),i) print("".join(list1)) reverse(user_input)Aniruddha Chaudhari
Interesting. But, it will consume extra memory for strings and lists.
Dwarakanath
Thank you for your response. Can you please guide in memory management and time complexity?
Aniruddha Chaudhari
In your example, you have created an extra list (list1) of leght n. So the space complexity has increased to O(n).
As you looping over the list only once, the time complexity is O(n) which is very efficient.
Birobrata Deb
How to get this output “gnirtS; eb2 desrever…” where the condition is:
1. Reverse each word in the input string.
2. The order of the words will be unchanged.
3. A word is made up of letters and/or numbers.
4. Other characters (spaces, punctuation) will not be reversed.
Viazovik Anton
how to collect words in advance and add them to the list already folded?
The main check must pass.
def reverse_string(str_smpl): lst = [] for word in str_smpl.split(' '): letters = [c for c in word if c.isalpha()] for c in word: if c.isalpha(): lst.append(letters.pop()) continue else: lst.append(c) lst.append(' ') print("".join(lst)) if __name__ == '__main__': cases = [ ('aa1b d3c 13sgf%', 'ba1a c3d 13fgs%'), ('a1bcd efg!h', 'd1cba hgf!e'), ('abcd efgh', 'dcba hgfe'), ('', '') ] for text, reversed_text in cases: assert reverse_string(text) == reversed_textk.gokulappadurai
Aniruddha Chaudhari
The code you shared is to reverse the string. We want to reverse the string without changing the positions of special characters.
Kamath
Can I reverse the string without changing the positions of special characters and without using string function like isalpha(), isalnum?
Aniruddha Chaudhari
Yes. It is possible doing it manually instead of using a built-in function.