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Rotate String – Solution & Complexity

Interview solutions
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Rotate String

Solution Walkthrough

  1. Step 1: Recognize the Pattern

    • Rotating a string does not change its length or multiset of characters.
    • More specifically, every rotation of s appears as a contiguous slice inside s + s.
    • So the problem reduces to checking equal lengths first, then substring containment.

  2. Step 2: Build the Algorithm

    • If s and goal have different lengths, return False immediately.
    • Otherwise form the doubled string s + s.
    • Return whether goal appears inside that doubled string.

  3. Step 3: Check Edge Cases

    • Identical strings should return True because zero rotations are allowed.
    • Single-character strings work naturally with the same check.
    • Repeated characters are safe because substring containment on s + s still distinguishes real rotations from lookalikes.

  4. Step 4: Solution and Complexity

    • The doubled-string trick captures every possible rotation exactly where it starts in s + s.
    • The algorithm runs in O(n) time for strings of length n and uses O(n) extra space for the doubled string.
    def rotate_string(s: str, goal: str) -> bool:
    return len(s) == len(goal) and goal in (s + s)

FAQ

See also

Practice Rotate String online (run code in the browser)Alien Dictionary solution (hard)Count Vowels solution (easy)Decode String solution (medium)Decode Ways solution (medium)All coding interview problems
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