I was contacted by phone two weeks after applying online. They asked me a question that had to be solved on a website called HackerRank. The question was very difficult; I still don't know how to solve it.

Given an array of characters, you must delete all characters that are repeated 3 or more times consecutively and add a space " " to the end of the array for every deleted character.

Examples: "aaabbbcddddd" -> "c " (11 spaces) "xxaaavbbbc" -> "xxvc " "xavvvarrrt" -> "xaaat "

The problem must be solved with:

• O(1) memory
• O(n) time

And you cannot overwrite a cell in the array more than once.