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?

 

考察一个数的二进制表示。将一个数表示成二进制会发现其每一位0不是就是1.题目描述说除了有一个数只出现了1次之外，其余的数都出现了3次。因此我们可以统计在二进制的每一位上0和1出现的次数，将次数mod3 则其结果就是single number 二进制表示时在这一位上的结果。

public class Solution {
    public int singleNumber(int[] nums) {
        int ans = 0;
        for (int i = 0; i < 32; i++) {
            int count = 0;
            for (int j = 0; j < nums.length; j++) {
                count += (nums[j] >> i) & 1;
            }
            if (count % 3 != 0) {
                ans |= 1 << i;
            }
        }
        return ans;
    }
}