[Lintcode]Singleton

Singleton is a most widely used design pattern. If a class has and only has one instance at every moment, we call this design as singleton. For example, for class Mouse (not a animal mouse), we should design it in singleton.

You job is to implement a 

getInstance

 method for given class, return
the same instance of this class every time you call this method.

new时要注意防止在并发情况下被new多次。

class Solution {
/**
* @return: The same instance of this class every time
*/
private static volatile Solution so = null;
private Solution() {}

public static Solution getInstance() {
if(so == null){
synchronized (Solution.class)
so = new Solution();
}
return so;
}
};