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

Have you met this question in a real interview? 

Yes

Example

In Java:

A a = A.getInstance();
A b = A.getInstance();

a should equal to b.

Challenge

If we call getInstance concurrently, can you make sure your code could run correctly?

class Solution {
    /**
     * @return: The same instance of this class every time
     */
   
     static Solution instance = null;
    public static Solution getInstance() {
        // write your code here
        if(instance == null){
            synchronized (Solution.class){
                if(instance == null)
                instance =  new Solution();
            }
        }
        return instance;
    }
};