Help needed in understanding this question, please.

Question

Here is the question:
What is the number displayed when the following program is compiled and run.
class test {
    public static void main(String args[]) {
        test test1 = new test();
            System.out.println(test1.xyz(100));   
    public int xyz(int num) {
        if(num == 1) return 1;
        else return(xyz(num-1) + num);
Answer= 5050.
I'll be honest I am lost as to what happens in this code. Any help in explaining what happens in the code would be gratefully received, thanks, Dave.

Answer 1

public int xyz(int num) {
if(num == 1) return 1;
else return(xyz(num-1) + num);
it's called recursion, the method calls itself
xyz(100) returns
xyz(99)+100 which returns
xyz(98)+99 which returns
xyz(97)+98 which returns
xyz(99)+100 which returns
until you get to xyz(1) which returns 1
so starting from the end of the recursion you get
(for num = 2): 1+2
(for num = 3): 3+3
(for num = 4): 6+4
(for num = 5): 10+5
(for num = 100): 4950+100