Longest Substring with K Distinct

Problem: Longest substring with at most $k$ distinct characters.

function lengthOfLongestSubstringKDistinct(s, k):
    charCount = empty map
    left = 0, maxLen = 0

    for right from 0 to s.length - 1:
        charCount[s[right]]++
        while charCount.size > k:
            charCount[s[left]]--
            if charCount[s[left]] == 0:
                charCount.remove(s[left])
            left++
        maxLen = max(maxLen, right - left + 1)
    return maxLen

Time: $O(n)$. Space: $O(k)$.