Leetcode #2981: Find Longest Special Substring That Occurs Thrice I
In this guide, we solve Leetcode #2981 Find Longest Special Substring That Occurs Thrice I in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
You are given a string s that consists of lowercase English letters. A string is called special if it is made up of only a single character.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Hash Table, String, Binary Search, Counting, Sliding Window
Intuition
Fast membership checks and value lookups are the heart of this problem, which makes a hash map the natural choice.
By storing what we have already seen (or counts/indexes), we can answer the question in one pass without backtracking.
Approach
Scan the input once, using the map to detect when the condition is satisfied and to update state as you go.
This keeps the solution linear while remaining easy to explain in an interview setting.
Steps:
- Initialize a hash map for seen items or counts.
- Iterate through the input, querying/updating the map.
- Return the first valid result or the final computed value.
Example
Input: s = "aaaa"
Output: 2
Explanation: The longest special substring which occurs thrice is "aa": substrings "aaaa", "aaaa", and "aaaa".
It can be shown that the maximum length achievable is 2.
Python Solution
class Solution:
def maximumLength(self, s: str) -> int:
def check(x: int) -> bool:
cnt = defaultdict(int)
i = 0
while i < n:
j = i + 1
while j < n and s[j] == s[i]:
j += 1
cnt[s[i]] += max(0, j - i - x + 1)
i = j
return max(cnt.values()) >= 3
n = len(s)
l, r = 0, n
while l < r:
mid = (l + r + 1) >> 1
if check(mid):
l = mid
else:
r = mid - 1
return -1 if l == 0 else l
Complexity
The time complexity is , and the space complexity is , where is the length of the string , and represents the size of the character set. The space complexity is , where is the length of the string , and represents the size of the character set.
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.