What is the time complexity of LeetCode 395. Longest Substring with At Least K Repeating Characters?

The Python solution on this page runs in O(n).

What is the space complexity of LeetCode 395. Longest Substring with At Least K Repeating Characters?

The Python solution on this page uses O(n) auxiliary space.

Longest Substring with At Least K Repeating Characters — LeetCode 395 Python Solution

MediumHash TableStringDivide and ConquerSliding Window

Problem: #395
Pattern: Sliding Window
Reading time: 4 min
Source: leetcode.com

The problem

Given a string s and an integer k, return the length of the longest substring of s such that the frequency of each character in this substring is greater than or equal to k. if no such substring exists, return 0.

Example

Input: s = "aaabb", k = 3
Output: 3
Explanation: The longest substring is "aaa", as 'a' is repeated 3 times.

Python solution

Python

class Solution:
    def longestSubstring(self, s: str, k: int) -> int:
        def dfs(l, r):
            cnt = Counter(s[l : r + 1])
            split = next((c for c, v in cnt.items() if v < k), '')
            if not split:
                return r - l + 1
            i = l
            ans = 0
            while i <= r:
                while i <= r and s[i] == split:
                    i += 1
                if i >= r:
                    break
                j = i
                while j <= r and s[j] != split:
                    j += 1
                t = dfs(i, j - 1)
                ans = max(ans, t)
                i = j
            return ans

        return dfs(0, len(s) - 1)

Complexity

Measure	Complexity
Time	O(n)
Space	O(n) auxiliary

Pattern: Sliding Window

Collapse a nested loop over every subarray into a single pass with two indices. LeetCode 395. Longest Substring with At Least K Repeating Characters is filed here because LeetCode tags it Sliding Window, which is the vocabulary this hub collects.

The sliding window guide has the Python template for the pattern and the 116 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 395. Longest Substring with At Least K Repeating Characters?: LeetCode 395. Longest Substring with At Least K Repeating Characters is rated Medium on LeetCode.
What is the time complexity of LeetCode 395. Longest Substring with At Least K Repeating Characters?: The Python solution on this page runs in O(n).
What is the space complexity of LeetCode 395. Longest Substring with At Least K Repeating Characters?: The Python solution on this page uses O(n) auxiliary space.
What topics does LeetCode 395. Longest Substring with At Least K Repeating Characters cover?: LeetCode 395. Longest Substring with At Least K Repeating Characters is tagged Hash Table, String, Divide and Conquer and Sliding Window on LeetCode.