Finding Unique number

Q: You are given an array that contains integers. The integers content is such that every integer occurs 3 times in that array leaving one integer that appears only once.
Fastest way to find that single integer

eg:
Input: {1,1,1,3,3,3,20,4,4,4};

Answer: 20

The logic is similar to finding the element which appears once in an array - containing other elements each appearing twice. There we do XOR all the elements and the result will be the unique number.

To apply the similar logic to this problem we maintain 2 variables

ones - This variable holds XOR of all the elements which have appeared only once.
twos - This variable holds XOR of all the elements which have appeared twice.

The algorithm goes as follows…

1. A new number appears - It gets XOR' ed to the variable "ones".
2. A number gets repeated(appears twice) - It is removed from "ones" and XOR'd to the
variable "twos".
3. A number appears for the third time - It gets removed from both "ones" and "twos".

The final answer will be in "ones" as it holds the unique element.

Working Java code: