Single Number II

Given 3*n + 1 numbers, every numbers occurs triple times except one, find it.

Example
Given [1,1,2,3,3,3,2,2,4,1] return 4

Challenge
One-pass, constant extra space.

public class Solution {
    /**
     * @param A : An integer array
     * @return : An integer 
     */
    public int singleNumberII(int[] a) {
        // write your code here
        if(a==null ||a.length ==0)return -1;
        int [] bits =new int [32];
        int result =0;
        for(int i=0;i<32;i++){
            for(int j =0;j<a.length;j++){
                bits[i]+=a[j]>>i&1;

            }
            bits[i]=bits[i]%3;
            result |=bits[i]<<i;
        }


        return result;


    }
}