137. Single Number II

题目：

Given an array of integers, every element appears three times except for one, which appears exactly once. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

链接：https://leetcode.com/problems/single-number-ii/#/description

6/10/2017

用extra space

 1 public class Solution {
 2     public int singleNumber(int[] nums) {
 3         Map<Integer, Integer> count = new HashMap<Integer, Integer>();
 4         
 5         for (int i = 0; i < nums.length; i++) {
 6             count.put(nums[i], count.getOrDefault(nums[i], 0) + 1);
 7         }
 8         for (Map.Entry<Integer, Integer> entry : count.entrySet()) {
 9             if (entry.getValue() == 1) {
10                 return entry.getKey();
11             }
12         }
13         return -1;
14     }
15 }

不会不用extra space的

很详细的解释，但是只能耐心看完m和k部分，p部分的分析没耐心看完了。他的后来的几个例子还是不错的。

https://discuss.leetcode.com/topic/11877/detailed-explanation-and-generalization-of-the-bitwise-operation-method-for-single-numbers?page=1