# LeetCode 917. Reverse Only Letters

## Description

https://leetcode.com/problems/reverse-only-letters/

Given a string `S`, return the “reversed” string where all characters that are not a letter stay in the same place, and all letters reverse their positions.

Example 1:

```Input: "ab-cd"
Output: "dc-ba"
```

Example 2:

```Input: "a-bC-dEf-ghIj"
Output: "j-Ih-gfE-dCba"
```

Example 3:

```Input: "Test1ng-Leet=code-Q!"
Output: "Qedo1ct-eeLg=ntse-T!"
```

Note:

1. `S.length <= 100`
2. `33 <= S[i].ASCIIcode <= 122`
3. `S` doesn’t contain `\` or `"`

## Explanation

Two pointers technique to swap alphabetical letters.

## Python Solution

``````class Solution:
def reverseOnlyLetters(self, S: str) -> str:
i = 0
j = len(S) - 1

S_list = list(S)

while i < j:
if S_list[i].isalpha() and S_list[j].isalpha():
temp = S_list[j]
S_list[j] = S_list[i]
S_list[i] = temp

i += 1
j -= 1
elif S_list[i].isalpha():
j -= 1
elif S_list[j].isalpha():
i += 1
else:
i += 1
j -= 1

return "".join(S_list)``````
• Time Complexity: O(N).
• Space Complexity: O(N).