Run-length encode in C#
C# interview exercise on Strings, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write Compress(s) that run-length encodes s: each run becomes the character followed by its length, and the length is left out when it is 1. Build the result with a StringBuilder: adding to a string in a loop copies it every time.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.