**Objective:** Given a string, write an algorithm to find the longest substring with at most K characters.

**Example:**

Input: aabbaacdeeeeddded, K = 3 Output: Longest substring with 3 most unique characters is: cdeeeeddded with length 11 Input: abcddefabc, K = 4 Output: Longest substring with 4 most unique characters is: abcdd with length 5 Input: aaaabbbb, K = 4 Output: Not enough unique character is present in the input string

**Approach: Brute Force**

Identify all the substrings of the given input string and check each substring with the number of unique characters in it and pick the one with maximum length.

**Time Complexity:** O(N^{3}), N^{2} for all finding the substrings and O(N) for each substring while checking unique characters.

**Better Solution – Linear Search **

- Check if the string has at least K unique characters if no then prints the error message and return.
- Let’s say we take substring between indexes curr_start and curr_end, both points to index 0 at the beginning.
- Keep adding one character at a time to the substring by moving curr_end pointer to the right and make sure that condition of substring has at most K unique characters , is valid by adding the new character and if any point the condition is not valid then start moving curr_start pointer to the right till condition is valid again.
- At each step, keep track of the length of a maximum substring by doing curr_end-curr_start.
- To check whether substring has only K unique characters, maintain an array of size 26 ( 26 alphabets) and the array is filled with the count of characters in the string to the respective index. 0th index for ‘a’ and 25th index for z.
- See the example below for more understanding.

**Time Complexity: O(N)**

Input: abacdedd, K = 3Processing character:a SubString: a a:1, curr_start: 0, curr_end: 1, max_Start: 0, max_Length: 1 ----------------------------------------------- Processing character:b SubString: ab a:1, b:1, curr_start: 0, curr_end: 2, max_Start: 0, max_Length: 2 ----------------------------------------------- Processing character:a SubString: aba a:2, b:1, curr_start: 0, curr_end: 3, max_Start: 0, max_Length: 3 ----------------------------------------------- Processing character:c SubString: abac a:2, b:1, c:1, curr_start: 0, curr_end: 4, max_Start: 0, max_Length: 4 ----------------------------------------------- Processing character:d SubString: abacd a:2, b:1, c:1, d:1, More than 3 unique characters in substring, moving curr_start to right curr_start: 1, curr_end: 5, max_Start: 0, max_Length: 4 a:1, b:1, c:1, d:1, More than 3 unique characters in substring, moving curr_start to right curr_start: 2, curr_end: 5, max_Start: 0, max_Length: 4 a:1, c:1, d:1, curr_start: 2, curr_end: 5, max_Start: 0, max_Length: 4 ----------------------------------------------- Processing character:e SubString: acde a:1, c:1, d:1, e:1, More than 3 unique characters in substring, moving curr_start to right curr_start: 3, curr_end: 6, max_Start: 0, max_Length: 4 c:1, d:1, e:1, curr_start: 3, curr_end: 6, max_Start: 0, max_Length: 4 ----------------------------------------------- Processing character:d SubString: cded c:1, d:2, e:1, curr_start: 3, curr_end: 7, max_Start: 0, max_Length: 4 ----------------------------------------------- Processing character:d SubString: cdedd c:1, d:3, e:1, curr_start: 3, curr_end: 8, max_Start: 3, max_Length: 5 Longest substring with 3 most unique characters is : cdedd with length 5

**Complete Code:**

**Output:**

Input: aabbaacdeeeeddded Longest substring with 4 most unique characters is : aacdeeeeddded with length 13