Think about what you do here and what you should.
mergesort(a, low, high);
mergesort(a, mid + 1, high);
merge(a, low, high, mid);
There is a tool that allow you to see what your code is doing, its name is debugger
. It is also a great learning tool because it show you reality and you can see which expectation match reality.
When you don't understand what your code is doing or why it does what it does, the answer is debugger
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.
Debugger - Wikipedia, the free encyclopedia
Mastering Debugging in Visual Studio 2010 - A Beginner's Guide
Basic Debugging with Visual Studio 2010 - YouTube
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.