Is it not possible to create a O(n) solution?

By creating a hashmap where the keys are the digits and the values are the number of times that the digits appear, and then looking for the hashmap value that is greater than n / 2?

By using a sort you also run the risk of using a bad sort algo that could be O(n²⁾ - not all languages make it clear which sort algo is actually used.

Link to problem: Majority Element.