mediumStringsPattern: Sliding Window
Longest Substring with K Unique Characters Solution
Problem Statement
Given a string and an integer k, find the length of the longest substring that contains exactly k unique characters.
Examples
Example 1:
Input:s = 'abcba', k = 2
Output:2
Explanation: The longest substrings with 2 unique characters are 'ab', 'bc', 'cb', and 'ba' with length 2.
Example 2:
Input:s = 'abcdabc', k = 3
Output:6
Explanation: The longest substring with 3 unique characters is 'abcdab' with length 6.
Constraints
- 1 <= k <= 26
- 1 <= s.length <= 10^5
- s consists of only lowercase English letters
Time: O(n) Space: O(k)
The optimized approach uses a sliding window technique with a frequency map to track the unique characters in the current window, achieving a time complexity of O(n).
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