L1 · FoundationsText basics~5 min · 6 tests

Caesar Cipher Encryption

Shift every letter of a message by k positions with wrap-around, keeping case and leaving other characters alone. Modular arithmetic in Python, tested.

The problem

Shift every letter in text forward by k positions in the alphabet, wrapping around (z → a). Keep upper/lower case; leave non-letters unchanged. k may be larger than 26 or negative.

Examples

Example 1
Input
```
caesar('abc', 1)
```
Expected output
```
'bcd'
```

Example 2

Input

caesar('Hello, World!', 3)

Expected output

'Khoor, Zruog!'

+ 4 hidden tests on Submit — wrap around, negative shift.

Edge cases to ask about

Wrap-around
Negative k
k > 26
Punctuation

Hints

0/3

How an interviewer scores this

0/9

1. Understand the problemRestate it, ask about edge cases and input size.
2. Write a brute-force solutionCorrect first, clever later.
3. Get the correct outputautoAll visible examples pass.
4. Analyse time complexityautoCommit to a Big-O in the Complexity Lab.
5. Analyse space complexityautoExtra memory, not the input.
6. Identify the bottleneckautoMeasure your code — where does it bend?
7. OptimiseUsually: a set or dict instead of a scan.
8. Explain why it worksSay it out loud to Solvi.
9. Test the edge casesautoSubmit passes every hidden test.

Python 3.13 · caesar

def caesar(text, k):
    """Shift letters by k, wrapping, preserving case."""
    # Write your solution here
    pass

⌘/Ctrl + Enter runs the examples

Your code runs in real CPython inside your browser — nothing is sent anywhere. The first run downloads the interpreter (about 6 MB, once). Your code is saved on this device as you type.

Complexity Lab

What does this cost as n grows?

Interviewers score the analysis as much as the code. Commit to an answer first — then check it, and read why.

Time complexity of the optimal solution

Space complexity (extra memory)

Pick both to reveal the answer.

From brute force to optimal

The progression an interviewer wants to hear, one step at a time.

Approach	Time	Space	Idea
ord/chr with modulo 26	O(n)	O(n)	Python's % is always non-negative for a positive divisor, so negative k works.

Walkthrough of the optimal approach (try it yourself first)

Map a letter to 0–25 (ord(ch) - ord('a')), add k, wrap with % 26, and map back with chr. In Python -1 % 26 == 25, so negative shifts work without special cases.

Complexity: O(n) time, O(n) space. Each character is converted once.

Reveal the reference solution

def caesar(text, k):
    out = []
    for ch in text:
        if "a" <= ch <= "z":
            out.append(chr((ord(ch) - ord("a") + k) % 26 + ord("a")))
        elif "A" <= ch <= "Z":
            out.append(chr((ord(ch) - ord("A") + k) % 26 + ord("A")))
        else:
            out.append(ch)
    return "".join(out)

Follow-ups interviewers ask

Write the decoder.
Break it without knowing k.

Frequently asked interview questions

Core interview concepts, complexities, and follow-ups scored by hiring teams.

What is the time complexity of Caesar Cipher Encryption in Python?

The optimal solution runs in O(n) time and O(n) auxiliary space. Each character is converted once.

What follow-up questions do interviewers ask about Caesar Cipher Encryption?

Write the decoder. Break it without knowing k.