Leetcode习题： SingleNumber II

Single Number II AC Rate: 323/1255My 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;    }}