Longest Substring with At Most K Distinct

Tested Python solution for LeetCode 340 with 15 pytest cases. Generate a practice environment with lcpy.

LeetCode 340, Medium. Topics: Hash Table, String, Sliding Window. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 15 parametrized pytest cases, and a playground notebook:

lcpy gen -n 340   # by problem number
lcpy gen -s longest_substring_with_at_most_k_distinct_characters   # by problem name

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Problem

Given a string s and an integer k, return the length of the longest substring of s that contains at most k distinct characters*.

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Examples

Input: s = "eceba", k = 2
Output: 3
Explanation: The substring is "ece" with length 3.

Input: s = "aa", k = 1
Output: 2
Explanation: The substring is "aa" with length 2.

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Constraints

• 1 <= s.length <= 5 * 10^4
• 0 <= k <= 50

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

class Solution:
    # Time: O(n) — each character enters and leaves the window once
    # Space: O(k) — counter holds at most k + 1 distinct characters
    def length_of_longest_substring_k_distinct(self, s: str, k: int) -> int:
        if k == 0:
            return 0

        counts: dict[str, int] = {}
        left = 0
        best = 0
        for right, ch in enumerate(s):
            counts[ch] = counts.get(ch, 0) + 1
            while len(counts) > k:
                left_ch = s[left]
                counts[left_ch] -= 1
                if counts[left_ch] == 0:
                    del counts[left_ch]
                left += 1
            best = max(best, right - left + 1)
        return best

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Complexity

Time	Space
O(n) — each character enters and leaves the window once	O(k) — counter holds at most k + 1 distinct characters

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Tags

NeetCode All.