leetcode--Single Number II

Given an array of integers, every element appears three times except for one. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

 

Have you been asked this question in an interview? 

/**We can use the bit operation to do this problem
*If you have a problem to understand the bit operation,
*then you can think | , & , ^, ~ as the set operation union, intersection and symmetric difference and set complement respectively.
*/

public class Solution {
    public int singleNumber(int[] A) {
        int once = 0;
        int twice = 0;
        int triple = 0;
        for(int i = 0 ; i < A.length ; i++){
            //update twice when A[i] is added. if A[i] is in once, then added it to twice,
            //otherwise, twice is not needed to updated     
            twice |= A[i] & once;
           //update once
            once = A[i] ^ once;
           //triple is exactly the intersection of once and twice
            triple = once&twice;
           //update once and twice again(remove all integer in triple) 
            once ^= triple;
            twice ^= triple;
        }
        //return the integer in once or twice
        return once | twice;
    }
}