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

A possible algorithm (very generic, not tested) :

function findMagicNumber(arr[0...n])
   magic_n := NaN

   if n = 1 then
      magic_n := arr[0]
   else if n > 1 then
      quicksort(arr)

      old_n := arr[0]
      repeat := 0

      for i := 1 to n
         cur_n := arr[i]
         repeat := repeat + 1
         if cur_n ≠ old_n then
            if repeat = 1 then
               magic_n := old_n
            old_n := cur_n
            repeat := 0

   return magic_n