面试题：找出数组中只出现一次的数字（二）

难度：中等

一个整数数组，除了一个数之外所有数字都出现了3次，找出这个数字来。

注意: 你的算法应该是线性运行复杂度，且不能使用额外内存空间。

答案：

public class Solution {
    public int singleNumber(int[] nums) {
        int ones = 0, twos = 0, threes = 0;

        for (int i = 0; i < nums.length; i++) {
          // twos holds the num that appears twice
          twos |= ones & nums[i];

          // ones holds the num that appears once
          ones ^= nums[i];

          // threes holds the num that appears three times
          threes = ones & twos;

          // if num[i] appears three times
          // doing this will clear ones and twos
          ones &= ~threes;
          twos &= ~threes;
        }

        return ones;
    }
}