Encrypt and Decrypt Strings
A hard-tier problem at 37% community acceptance, tagged with Array, Hash Table, String. Reported in interviews at Duolingo and 0 others.
Encrypt and Decrypt Strings is a hard problem asked by Duolingo with a 37% acceptance rate. You're building a stateful encoder-decoder system that has to handle collisions and maintain bidirectional mapping. The trap is thinking simple substitution works. Once you add even a few encrypt calls, your naive approach crumbles under collisions or deserialization failures. This is the kind of design problem where the obvious path fails halfway through the assessment. If you hit it cold and can't thread the state management together fast enough, StealthCoder surfaces a working solution invisible to the proctor.
Companies that ask "Encrypt and Decrypt Strings"
Encrypt and Decrypt Strings is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage.
Get StealthCoderThe core trick is that you can't just map strings to fixed-length codes. You need a scheme that's unambiguous when you concatenate encrypted results. Most candidates start with a counter and hit collisions immediately. The real solution uses a Trie for efficient encoding, careful handling of string boundaries during decryption, and a Hash Table to track what's been encrypted so far. The pattern involves understanding why some encoding schemes are reversible and others aren't, especially when the input is a concatenated stream of encrypted tokens. Common pitfalls include forgetting that decryption happens on a single concatenated string, not individual tokens, and not accounting for the need to track state across multiple encrypt and decrypt calls. StealthCoder is your safety net if you freeze on the state-management piece.
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Encrypt and Decrypt Strings recycles across companies for a reason. It's hard-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Encrypt and Decrypt Strings interview FAQ
Why does a simple number-to-string mapping fail here?+
If you map "hello" to "1" and "hell" to "2", how do you decode "12". Is it "hell"+"hell" or "hello"+"2". You need delimiters or fixed-width codes, which then require careful design. The problem forces you to think about boundary detection during decryption.
What's the role of Trie in this problem?+
A Trie helps you build an efficient, hierarchical encoding scheme and speeds up lookups during decryption. It's not mandatory, but it's the structure that naturally handles the prefix-matching needed when you're decoding a concatenated string of encrypted tokens.
How do you handle the stateful nature of encryption?+
You track what strings have been encrypted and assign them new codes in order. The encoder increments a counter, the decoder must reverse that mapping. Hash tables store the bidirectional mapping (string to code and code to string) so both operations are O(1).
Is this still asked at real companies, or just Duolingo?+
Duolingo reports it. It's a less common design problem compared to LRU cache or type-ahead systems. Hard problems at this acceptance rate usually have narrow company ask, but the pattern is relevant to state management in any system that encodes data.
What's the biggest gotcha during live assessment?+
Candidates often build encryption fast, then realize decryption on a concatenated string breaks their scheme. You need to test both operations together early. Off-by-one errors in boundary detection are common. The state management also trips people up if they don't reset counters or mappings correctly between test cases.
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