What is the difference between singleton and static class?

What is the difference between singleton and static class?

These are just my opinion. 

1. You can use an interface class. In other words, your singleton class can implement interface functions.

2. As you know, memory area is different. A object is stored in heap area and static class is placed in stack. For memory optimization, the singleton way could be useful. 

3. In a static class, you can set member variables at any time. But in a singleton way, you can set the values only after creating an instance. So, the restriction of singleton way could be useful.