Print Singly linked list in reverse using recursion - Java -


hi having little trouble trying print singly linked list in reverse order using recursion. have looked @ examples method doesn't take parameters. want print out in following format:

input: [1, 2, 3, 4, 5] , output:[5, 4, 3, 2, 1]

first refers to first node in singly linked list , use stringbuilder build list can return @ end.

this have far:

public string printreverse() {     stringbuilder mystring = new stringbuilder("[");     if (head != null) { // base case         head = head.next;         mystring.append(head.value);   // line 406         mystring.append(", ");         // line 407         printreverse();                // line 408     }     mystring = mystring.append("]");     return mystring.tostring(); }

i following error:

exception in thread "main" java.lang.nullpointerexception     @ myprog.sll$node.access$100(sll.java:445)      @ myprog.sll.printreverse(sll.java:406)      @ myprog.sll.printreverse(sll.java:408)      @ myprog.sll.printreverse(sll.java:408)      @ myprog.sll.printreverse(sll.java:408)      @ myprog.sll.printreverse(sll.java:408)      @ myprog.sllapp.mymethod(sllapp.java:198)      @ myprog.sllapp.<init>(sllapp.java:37)      @ myprog.sllapp.main(sllapp.java:26)

i don't see doing wrong, suspect may way call method on itself. can suggest may doing wrong , how may go fixing it?

thanks!

you on complicating things. lets @ pseudo code:

  • initial node head
  • if next null print blank (recursion termination condition)
  • else recurse next node
  • then print current node

in code, becomes:

public string printreverse() {     return printreverse(head);  }  private string printreverse(node n) {     return next == null ? "" : (printreverse(next) + n.value); }

it's 2 lines of code - see kiss.

regarding second private method, common public method of recursive implementation set ip call private recursive method appropriate initial state.


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