Detect permutation in string
Last updated: April 20, 2026
Quick Overview
Given two strings, s1 and s2, determine if s2 contains any permutation of s1 as a substring. The function should return true if such a permutation exists, and false otherwise. The input strings consist of lowercase English letters only, and the solution should efficiently handle cases where the lengths of the strings are large.
HRT
April 20, 202643
11
772 solved
Given two strings, s1 and s2, determine if s2 contains any permutation of s1 as a substring. The function should return true if such a permutation exists, and false otherwise. The input strings consist of lowercase English letters only, and the solution should efficiently handle cases where the lengths of the strings are large.
Coding interviews at HRT focus on problem-solving approach as much as the final solution. The interviewer wants to see you break down the problem, consider edge cases, and optimize iteratively. Communication throughout the process is key.
What the Interviewer Expects
- Quickly identify the optimal approach and its theoretical basis
- Handle complex algorithm design with multiple interacting components
- Write concise, elegant code under time pressure
- Prove correctness of your approach and discuss alternative solutions
- Optimize beyond the obvious: discuss constant factor improvements
- Address follow-up variations and explain how the solution generalizes
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- What if the input doesn't fit in memory?
- Can you optimize the space complexity of your solution?
- How would your solution change if the input was sorted?
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Practice DSA ProblemsSample Answer
Problem Analysis
To determine if string s2 contains any permutation of string s1 as a substring, we can leverage the properties of permutations. A permutation of s1 will have the same character frequency as s1...
Approach
- First, create a frequency count (hashmap) for
s1that records how many times each character appears. - Initialize a sliding window of size equal to the length of
s1ons2. - For the first w...
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