Reverse arraylist

Question

Hi every one....

How can i reverse an arraylist in java without using any methods....
Like
Collections.reverse () ; and etc...

Answer 1

Denise Roßberg Yeah i do know that, but i am talking about the way of thinking. His way is the standard way of reversing an array, yours takes O(n) but still it is creative ^ ^

Answer 2

Honestly, just use in-builts. Usually they're built by smarter people who could do it better than you could.

Answer 3

You don't need a second list.

for(int i = list.size() - 1; i >= 0; i--){

//add the value to the list
list.add(list.get(i));
//remove value
list.remove(i);

}

E.g. 1 2 3 4

index = 3, value = 4
1 2 3 4 4 (add 4 & remove 4 at index 3)
-> 1 2 3 4

index = 2, value = 3
1 2 3 4 3 (add 3 & remove 3 at index 2)
-> 1 2 4 3

index = 1, value = 2
1 2 4 3 2 (add 2, remove 2 at index 1)
-> 1 4 3 2

index = 0, value = 1
1 4 3 2 1 (add 1 && remove 1 at index 0)
-> 4 3 2 1 -> done

Answer 4

List reversing using variable value swap method:
https://code.sololearn.com/c3yfniOgu249/?ref=app

Answer 5

Denise Roßberg UwU That's cool algorithm 😍

Answer 6

You can make a new list:
Arraylist newlist = new Arraylist()

for (int i = list.size() -1 ; i >= 0; i --){
newlist.add(list.get(i));
}

But maybe there are better solutions

Answer 7

,.., Thanks. But Seb TheS code is better.

Look at the 1 2 3 4 example. With my solution you need 4 steps.

But swapping the elements needs only 2 steps:
4 2 3 1
4 3 2 1

Answer 8

Umar