Challenge question in Java about recursion

Question

I found this question in Java challenge really strange. It's about recursion obviously, but after x becomes 100, I don't understand what happen next and how the answer is 95. If someone can explain I'll be grateful.
https://code.sololearn.com/c1Ss5wKJvyPU/?ref=app

Answer 1

TheWh¡teCat 🇧🇬

f(60) calls f(70) calls f(80) calls f(90) calls f(100)

Each function call goes into a stack:
f(90)
f(80)
f(70)
f(60)

f(100) returns 99
f(90) gets 99 from f(100) and returns 99 - 1
f(80) gets 98 from f(90) and returns 98 - 1
f(70) gets 97 from f(80) and returns 97 - 1
f(60) gets 96 from f(70) and returns 96 - 1
stack is empty -> output: 95

Answer 2

It's quite tricky, but it becomes easier to understand once you've written it down.

f(60)
= f(60+10) - 1 // f(70) - 1
= f(70+10) - 1 - 1 // f(80) - 2
= f(80+10) - 2 - 1 // f(90) - 3
= f(90) - 3 - 1 // f(100) - 4
= (100 - 1) - 4 = 95.

Answer 3

TheWh¡teCat 🇧🇬 Sankalp👨‍💻👨‍💻

Yes.

You can imagine a stack like a glass: the stuff which goes as first into stack is at the bottom.

|3|
|2|
|1|

When you want to empty a stack, its starts at 3. then 2, then 1.
-> LIFO last in, first out

I added some print statements to your code:
https://code.sololearn.com/ckFHWmZcXhQK/?ref=app

Edit: about stacks

https://www.sololearn.com/learn/642/?ref=app

Answer 4

Thanks a lot for the explanation Denise Roßberg , I think that solves the problem 🐱

Answer 5

Sankalp👨‍💻👨‍💻 , the exact explanation. I was debugging it few days ago and I was confused with the things I saw => on one hand the current argument of the method and next what returns the method five times 99, 80, 98, 70 and I didn't find the logic in this sequence. Now everything is clear 🐱

Answer 6

Thanks again for the detailed explanation, Denise Roßberg 🐱

Answer 7

Thanks for the effort Diego, but it doesn't seem to work in that pattern. I tried to debug it some days ago and if I remember it right (I'll check it again later) the recursion repeats until x becomes 100, then x is 99 (x - 1), but after that I just can't understand what happen - x becomes 88, next 70 and the next steps I don't remember.

Answer 8

TheWh¡teCat 🇧🇬 Your welcome :)