Find the Encrypted String - Solution & Explanation

Problem Overview: You receive a string s and an integer k. Each character in the encrypted string comes from index (i + k) % n of the original string, where n is the length of s. The task is to construct the resulting string after this cyclic shift.

Approach 1: Cyclic Reordering with Modulo (Time: O(n), Space: O(n))

The direct solution builds a new string by iterating through every index i from 0 to n-1. For each position, compute the source index using (i + k) % n and append that character to the result. The modulo operation handles wrap‑around when the index exceeds the string length. This approach is straightforward, avoids complex pointer manipulation, and works in linear time because each character is processed exactly once. It relies on basic string traversal and modular arithmetic.

Approach 2: Rotation with In-place Modification (Time: O(n), Space: O(1))

The encryption rule effectively performs a left rotation of the string by k positions. Instead of creating a new string, you can rotate the characters in place using the classic three‑reversal technique. First normalize k using k % n. Reverse the first k characters, reverse the remaining n-k, then reverse the entire sequence. After these operations, the characters appear in the same order as the encrypted result. This method is useful when memory usage matters or when you want to practice in-place manipulation techniques common in array and string rotation problems.

Recommended for interviews: The cyclic modulo approach is usually what interviewers expect first because it directly follows the formula in the problem statement and clearly demonstrates understanding of cyclic indexing. Mentioning the in-place rotation variant shows deeper familiarity with string/array manipulation and space optimization techniques.