find the largest distance between characters.

To find the largest distance between occurrences of a character `x` in a string str, where the distance is defined as the number of unique characters between two occurrences of x, you can iterate through the string while maintaining a set to track unique characters and an integer to track the maximum distance. Here's a step-by-step approach:

1. Initialize an integer variable `maxDistance` to 0. This will hold the maximum distance found.

2. Initialize an integer variable `lastPosition` to -1. This will store the last position where `x` was found.

3. Initialize an empty set uniqueChars. This will store the unique characters between two occurrences of x.

4. Iterate through the string. For each character:

   • If the character is not x, add it to uniqueChars.

   • If the character is x:

     • If `lastPosition` is not -1 (meaning this is not the first `x), calculate the distance from the last x` by getting the size of uniqueChars. If this distance is greater than maxDistance, update maxDistance.

     • Reset `uniqueChars` to an empty set.

     • Update `lastPosition` to the current position.

5. Return `maxDistance` as the result.

Here's how you can implement this algorithm in Python:

def largest_distance(str, x):

    max_distance = 0

    last_position = -1

    unique_chars = set()

    

    for i, char in enumerate(str):

        if char != x:

            unique_chars.add(char)

        else:

            if last_position != -1:

                distance = len(unique_chars)

                max_distance = max(max_distance, distance)

            unique_chars = set()

            last_position = i

            

    return max_distance

\# Test the function

str = "aaxbbyccxdd"

x = 'x'

print(largest_distance(str, x))  # Output should be the largest distance between two 'x's

This function iterates through the string once, making its time complexity O(n), where n is the length of the string. The space complexity is O(m), where m is the maximum number of unique characters between two occurrences of x.