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Find the Encrypted String — Easy Problem & Solution

Encrypt s by replacing every character with the character k places after it, wrapping around to the start of the string when you run off the end.

  • Difficulty: Easy
  • Topics: Strings, Simulation
  • Asked at: Amazon, Google, TCS
  • Time limit: 2 s
  • Memory limit: 256 MB
  • Languages: JavaScript, TypeScript, Python, Java, C++, C, C#, Go, Kotlin, Swift, Rust, PHP and Ruby

Problem statement

Encrypt s by replacing every character with the character k places after it, wrapping around to the start of the string when you run off the end.

Return the encrypted string.

Example 1

Input: s = "codekairo", k = 3
Output: ekairocod
Explanation: Position 0 takes the character at position 3, and so on around the ring.

Example 2

Input: s = "dart", k = 3
Output: tdar

Example 3

Input: s = "aaa", k = 1
Output: aaa
Explanation: Every character is the same, so rotating changes nothing.

Constraints

  • 1 <= s.length <= 100
  • 1 <= k <= 10^4
  • s consists only of lowercase English letters.

How to solve Find the Encrypted String

The answer at index i is s[(i + k) mod n] — a left rotation by k mod n. Build the result index by index.

Approach

  1. For each i from 0 to n - 1, append s[(i + k) mod n].
  2. Return the accumulated string.

Why it works

The modulo is doing all the work: k can be a hundred times the string's length, and taking it modulo n collapses every one of those wraps into the single rotation they amount to. Rotating one character at a time in a loop would be O(n · k) and needlessly slow for large k.

Complexity

  • Time — O(n)
  • Space — O(n) for the output

Pitfalls

  • Shifting is on positions, not on the letters themselves — this is not a Caesar cipher.
  • k may be far larger than the string; reduce it with mod n.
  • The direction is forward: position i reads from i + k, not i - k.

Reference solution

Python

def getEncryptedString(s: str, k: int) -> str:
    n = len(s)
    return "".join(s[(i + k) % n] for i in range(n))

JavaScript

var getEncryptedString = function(s, k) {
    var n = s.length, out = "";
    for (var i = 0; i < n; i++) out += s.charAt((i + k) % n);
    return out;
};

Also on the editorial tab: C, C#, C++, Go, Java, Kotlin, PHP, Ruby, Rust, Swift, TypeScript.

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