# [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.

1. abhishek says:

Hi,

How to do code for below one?

```def reverse_each_word(sentence):
# TODO: Implement this function
return
def main():
test_str =  "String; 2be reversed..."
assert reverse_each_word(test_str) ==  "gnirtS; eb2 deserved..."
return 0
```

Sample Input:

```Input string = "String; 2be reversed..."
Output: "gnirtS; eb2 deserved..."
```
1. 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.

2. Milan says:

Hi Abishek, do you got the solution for that?

2. rahul says:

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

1. Rahul, the Program you have written is to reverse each individual word in the sentence.

It returns.

`;gnirtS eb2 ...desrever`
3. Aashish says:

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:

`gnirtS; 2eb deserved...`
1. Birobrata Deb says:

An additional space gets appended at the end of the reversed string: “gnirtS; 2eb desrever… “

2. Amol says:

4. Santosh says:

Hi,
How do I include assert in the main() condition which must succeed upon error?

5. Dwarakanath says:
```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)
```
1. Dwarakanath says:

Thank you for your response. Can you please guide in memory management and time complexity?

1. 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.

6. Birobrata Deb says:

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.

7. Viazovik Anton says:

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_text
```
```#simply
str=input()
print(str[::-1])
```
1. The code you shared is to reverse the string. We want to reverse the string without changing the positions of special characters.

1. Kamath says:

Can I reverse the string without changing the positions of special characters and without using string function like isalpha(), isalnum?

9. Rahul says:

Bro how do we revere the string without changing the position.
Ex :

```#input = 'my name is rahul'
#output = 'ym eman si lu'
```
1. Rahul says:
```#input = 'my name is rahul'
#output = 'ym eman si luhar
```
1. Anonymous says:

let input=”my name is rahul”
let output=input.split(” “)

let final=””
for (i=0;i=0; j–){
reverse += output[i][j]
}
final+= reverse
}

console.log(final)

10. Rameshwar Pawar says:

pls, answer this code in java language.