Reverse Only Letters

Given a string s, reverse the string according to the following rules:

• All the characters that are not English letters remain in the same position.
• All the English letters (lowercase or uppercase) should be reversed.

Return s after reversing it.

Example 1:

Input: s = "ab-cd"
Output: "dc-ba"

Example 2:

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

Example 3:

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

Constraints:

• 1 <= s.length <= 100
• s consists of characters with ASCII values in the range [33, 122].
• s does not contain '\"' or '\\'.

Solution:

Here we will follow two pointer approach. Left and right pointer.

Left pointer will keep increasing and right pointer will keep decreasing till they cross the path except one condition ( If character is not a letter).

Complexity: Big O(N)

public class Solution {
    public string ReverseOnlyLetters(string s) {
        char[] ch = s.ToCharArray();
      
     // Two Pointer approach.
        int left = 0;
        int right = ch.Length-1;
      //Check if string has only one character.
        if(left == right)
            return s;
        
        while(left<= right)
        {
            if(Char.IsLetter(ch[left]) && Char.IsLetter(ch[right]))
            {
                     char temp = ch[right];
                     ch[right]= ch[left];
                     ch[left] = temp;
                     left++;
                     right--;
            }
            if(!Char.IsLetter(ch[left]))
            {
                left++;
                
            }
             if(!Char.IsLetter(ch[right]))
            {
                right--;
                
            }
}
        return new string(ch);
        
    }
}