137. Single Number II

Given an array of integers, every element appearsthreetimes 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?

思路：用HashSet

public class Solution {
    public int singleNumber(int[] nums) {
        LinkedHashMap<Integer,Integer> map = new LinkedHashMap<Integer,Integer>();
        for (int i=0;i<nums.length;i++){
            int a = nums[i];
            if (map.containsKey(a)){
                int Val = map.get(a);
                if (Val==1){
                    map.put(a,map.get(a)+1);
                }else if(Val==2){
                    map.remove(a);
                }
            }else{
                map.put(a,1);
            }
        }
        Iterator<Integer> it = map.keySet().iterator();
        int me = it.next();
        return me;
    }
}