topic description
 output an array of elements whose occurrence times are greater than n 
 add a findDuplicate (n) method for all array objects to return a list of elements whose occurrence frequency > = n in the array 
 [1, findDuplicate (2) = > [1] 
 [1, findDuplicate (5) = > [] 
 [1, 2, 3, 4, 4, 1, 2, 2, 2] = > findDuplicate (- 1) = > [1, 2, 3, 4, 4] 
