LeetCode 3090: Maximum Length Substring With Two Occurrences

Meta Data

#

Difficulty: easy First Attempt: 2026-01-10

• Submissions: Accepted (first try)

Approach

#

A window is valid ⇔ every character appears at most twice. Each time the right boundary adds a character, only that character can exceed the limit, so shrink the left until it is back to two.

LeetCode 2958 is the same problem with k in place of 2.

My Solution

#

class Solution:
    def maximumLengthSubstring(self, s: str) -> int:
        cnt = Counter()
        left = best = 0
        for right, c in enumerate(s):
            cnt[c] += 1
            while cnt[c] > 2:                      # only the new c can exceed
                cnt[s[left]] -= 1
                left += 1
            best = max(best, right - left + 1)
        return best

Complexity

#

Time O(n), space O(1) (at most 26 characters).

Mistakes to Avoid

#

• Checking the whole count table every time: only the newly added character can break the rule.

Interview Explanation

#

A longest-window slide where each character appears at most twice. After adding a character on the right, only it can exceed two, so I shrink the left until it is back to two and track the longest length. Time O(n).