Explain Operating System working: OS works as an intermediate between the user and computer. It helps the user to communicate with the computer without knowing how to speak the computer's language. The kernel is the central component of a computer operating systems.
In recursion method, a method calls itself until a condition becomes true and returned back. In mystery(), at first time, first is 0 and last is 4. Condition is checked and 0!=4 so else part is executed in which again mystery is called with first as 1 and last 4. This will go again and again until first=4.
When first=4 it is also equal to last so if part is executed and alpha[4] is returned which is 9, now it comes alpha[3] +9 which is 8+9 =17 .
It is returning values to its previous calls that is why it will reduce to alpha[0]+alpha[1]+alpha[2]+17 which is nothing but sum of all elements of a alpha
