Today’s exercise is in two parts, first a commonly-seen programming exercise and then a variant on it; the origin of the exercise is certainly someone’s homework, but since school is out for the year it doesn’t matter that we do the exercise today.

First, write a program that, given an array of integers in unsorted order, finds the single duplicate number in the array. For instance, given the input [1,2,3,1,4], the correct output is 4.

Second, write a program that, given an array of integers in unsorted order, finds all of the multiple duplicate numbers in the array. For instance, given the input [1,2,3,1,2,4,1], the correct output is [1,2,1].

Your task is to write the two programs that find duplicates in an array. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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