Is Unique Substrings in Wraparound String easy or hard?

The problem is rated Medium on LeetCode with an acceptance rate around 40–45%. The challenge comes from recognizing the counting trick that tracks only the longest valid substring ending at each character instead of generating all substrings.

Unique Substrings in Wraparound String Python/Java solution

Python, Java, C++, and JavaScript implementations follow the same pattern: iterate through the string, track the current wraparound length, and update a 26‑element array for each character. After processing the string, sum the array values to obtain the total number of unique substrings. The algorithm runs in O(n) time and constant space across all languages.

How to solve Unique Substrings in Wraparound String in O(n)?

Traverse the string while tracking the length of the current wraparound substring. If two adjacent characters follow the alphabet sequence (including 'z' to 'a'), increase the current length; otherwise reset it to 1. Store the maximum length for each ending character in a 26‑element array. Summing these values counts all unique substrings in linear time.

What is the best approach for Unique Substrings in Wraparound String?

The optimal approach uses dynamic programming with substring tracking. Maintain a 26‑length array where each entry stores the maximum length of a valid wraparound substring ending with that character. As you scan the string, extend the current sequence when characters follow the wraparound rule and update the array. The sum of all values gives the number of unique substrings in O(n) time and O(1) space.

Is Unique Substrings in Wraparound String asked at Google/Amazon/Meta?

This problem appears frequently in interview preparation sets for companies like Google, Amazon, and Meta because it tests string pattern recognition and dynamic programming optimization. The key skill evaluated is identifying how to avoid enumerating all substrings by tracking only the longest valid sequence per character.

What data structure is used in Unique Substrings in Wraparound String?

The solution mainly uses a fixed‑size array (length 26) to track the maximum wraparound substring length ending with each character. Combined with simple string traversal, this acts like a dynamic programming table. No complex structures such as trees or hash maps are required.

What is the time complexity of Unique Substrings in Wraparound String?

The optimal solution runs in O(n) time where n is the length of the input string. Each character is processed once while maintaining a constant‑size array of 26 letters. Space complexity is O(1) because only fixed alphabet tracking is required.

Description Hints 1 Solution Video 28:54

467. Unique Substrings in Wraparound String

Medium44.0% acceptance2 companies

We define the string base to be the infinite wraparound string of "abcdefghijklmnopqrstuvwxyz", so base will look like this:

"...zabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcd....".

Given a string s, return the number of unique non-empty substrings of s are present in base.

Example 1:

Input: s = "a"
Output: 1
Explanation: Only the substring "a" of s is in base.

Example 2:

Input: s = "cac"
Output: 2
Explanation: There are two substrings ("a", "c") of s in base.

Example 3:

Input: s = "zab"
Output: 6
Explanation: There are six substrings ("z", "a", "b", "za", "ab", and "zab") of s in base.

Constraints:

1 <= s.length <= 10⁵
s consists of lowercase English letters.

Asked at 2 companies

Maq Software

Google

Topics

Dynamic Programming String

Hints open one at a time

Hint 1

One possible solution might be to consider allocating an array size of 26 for each character in the alphabet. (Credits to @r2ysxu)

28:54

Still stuck?Read 3 approaches, or watch a walkthrough.

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1class Solution:
2    def findSubstringInWraproundString(self, s: str) -> int:
3        

Starter codePython 3Spaces: 4

Examples

Case 1Case 2Case 3

Input

"a"

Input

"cac"

Input

"zab"

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