Longest Substring with At Most K Distinct Characters

Given a string s and an integer k, return the length of the longest substring of s that contains at most k distinct characters. Example Input: s = "eceba", k = 2 Output: 3 Explanation: The longest valid substring is "ece". Constraints (typical) 0 <= len(s) <= 2e5 0 <= k <= 256 s consists of ASCII letters (or lowercase letters; clarify in interview). Example Input eceba 2 Output 3