Implement encryption and constant-time random set

Q: Implement encryption and constant-time random set

This question evaluates understanding of symmetric shared-secret string transformation and the design of a randomized set supporting expected O(1) operations, testing skills in cryptographic string manipulation and data-structure algorithmic reasoning.

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You are asked about two coding topics from an onsite interview:

Key-based string encryption Briefly explain the idea of shared-secret encryption: both sides use the same secret key, and the key determines how plaintext is transformed into ciphertext. Then implement a simple encryption function to make the task precise:
- Input: a plaintext string s of lowercase English letters and a non-empty lowercase key k
- Encryption rule: repeat the key as needed. For each character s[i] , shift it forward in the alphabet by k[i % k.length] - 'a' positions, wrapping around from z to a
- Output: the encrypted string
Example:
- s = "attack" , k = "dog"
- Output: "dhzdqq"
Randomized set with O(1) operations Design a data structure for unique integers that supports the following operations in average O(1) time:
- insert(val) : add val if it is not already present; return whether the insertion happened
- remove(val) : remove val if it is present; return whether the removal happened
- getRandom() : return a uniformly random element currently in the set

Implement encryption and constant-time random set

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