Counters are effectively building a sparse histogram by going through each element in x, and then each element in y. Ignoring the test for equality, the Big-O

Counters are effectively building a sparse histogram by going through each element in x, and then each element in y. Ignoring the test for equality, the Big-O for histograms is O(x+y). For sorting in general, the Big-O is O(x·log(x)+y·log(y)). In other words: the sorting solution is correct but less efficient.