Longest Substring with At Most Two Distinct

Tested Python solution for LeetCode 159 with 19 pytest cases. Generate a practice environment with lcpy.

LeetCode 159, Medium. Topics: Hash Table, String, Sliding Window. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 19 parametrized pytest cases, and a playground notebook:

lcpy gen -n 159   # by problem number
lcpy gen -s longest_substring_with_at_most_two_distinct_characters   # by problem name

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Problem

Given a string s, return the length of the longest substring that contains at most two distinct characters.

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Examples

Input: s = "eceba"
Output: 3
Explanation: The substring is "ece" which its length is 3.

Input: s = "ccaabbb"
Output: 5
Explanation: The substring is "aabbb" which its length is 5.

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Constraints

• 1 <= s.length <= 10^5
• s consists of English letters

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Solution

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

class Solution:
    # Time: O(n)
    # Space: O(1)
    def length_of_longest_substring_two_distinct(self, s: str) -> int:
        count: dict[str, int] = {}
        left = 0
        longest = 0
        for right, ch in enumerate(s):
            count[ch] = count.get(ch, 0) + 1
            while len(count) > 2:
                left_ch = s[left]
                count[left_ch] -= 1
                if count[left_ch] == 0:
                    del count[left_ch]
                left += 1
            longest = max(longest, right - left + 1)
        return longest

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Complexity

Time	Space
O(n)	O(1)

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Tags

NeetCode All.