Description：

Given an array of integers, every element appears twice except for one. Find that single one.

找出只出现一次的数。

public class Solution {    public int singleNumber(int[] nums) {        Map
     
       map = new HashMap
      
       ();                for(int i=0; i
       
         set = map.keySet();        for(int i : set) {            if(map.get(i) == 1) {                return i;            }        }        return 0;            }}