NullPointerException Error when printing Listnode val after reversing linkedlist

Question

You can see the problem in the attached picture. I also tried the print statement, "System.out.println(palindrome.val);" and I'm getting the same error. Please help me fix this. Thanks

click here to see the program

Answer 1

In the reverse function you are looping while head is not equals to null. After the loop you are returning head, which will definitely be null when the loop breaks.

So palindrome is null every time. And so doing palindrome.val gives NullPointerException.

To avoid this you can simple return the prev node from the reverse function.