Leetcode习题： SingleNumber II

Single Number II

AC Rate: 323/1255
My Submissions

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?

看有做法采用一个33位的数组来保存每一bit上3的次数，然后每次模3以满3时清零。然而这其实不满足题目"without using extra memory"的要求。还是搜索到了一种更为巧妙的解法。

public class Solution {

/**
* 其中ones记录了所有出现了模3余1次的bit，twos记录的是余2的。not_threes是当一个bit出现第3次的时候，把它清掉。
* 最后输出ones（如果题目中那个特殊的数出现了1次，当然如果是出现2次的话，应该输出twos
*
* @param A
* @return
*/
public int singleNumber(int[] A) {
int once = 0;
int twice = 0;

for (int i = 0; i < A.length; i++) {
twice |= once & A[i];
once ^= A[i];
int not_three = ~(once & twice);
once = not_three & once;
twice = not_three & twice;
}
return once;
}
}