(easy)

An array of integers, of odd size (int *array[2N+1]), is such that each number in the array appears twice, apart from one (the non-twin).

Problem: find an algorithm accepting the array as input and returning the value of the non-twin

Problem 2: same question with a second non-twin in the array; the algorithm should return both of them.

Another insight problem: Petals around the Rose.