Find a special number in an array

Question

There are many numbers in an array and each number appears three times excepting for one special number appearing once. Here is the question: how can I find the special numb

Answer 1

If the array is sorted, the problem is trivial, you just loop through the list, three items at a time, and check if the third item is the same as the current.

If the array is not sorted, you can use a Hash Table to count the number of occurences of each numbers.