Reverse a LinkedList in JAVA

icon1 Posted by Ankur Shah in Programming on 08 1st, 2011 | no responses

It’s 2AM and my insomnia is in full-motion as usual. My random browsing stumbled me upon this site that has a list of good technical/programming questions, and one of which, rehashes some of the memories from my college days. The task is to reverse a linked list, and while the question is asked to be implemented in C, I figured I’d take a stab at it in JAVA (just coz). It’s pretty straight forward, but if anyone has anything to add/comment, I’d like to hear:


package com.ashah.test;

public class ReverseLL {
    static class Node {
        public int nValue = 0 ;
        public Node next = null ;

        public Node(int n) { nValue = n ; }
        public String toString() { return "[" + nValue + "]" ; }
    }

    private static void printLL(Node head) {
        if (null != head) {
            System.out.print("{") ;
            while (null != head) {
                System.out.print(head) ;
                head = head.next ;
                if (null != head) {
                    System.out.print("->") ;
                }
            }
            System.out.println("}") ;
        }
    }

    private static Node reverseLL(Node head) {
        Node next = null ;
        Node previous = null ;
        while (null != head) {
            // ...keep next since we trash the next pointer
            next = head.next ;

            // ...switch the next pointer to point backwards
            head.next = previous ;

            // ...move both pointers forward
            previous = head ;
            head = next ;
        }

        return previous ;
    }

    public static void main(String[] args) {
        java.util.LinkedList ll = new java.util.LinkedList() ;
        Node n = null ;
        for (int i=1; i<=10; ++i) {
            n = new Node(i) ;
            if (i != 1) {
                ((Node)ll.get(i-2)).next = n ;
            }
            ll.add(n) ;
        }
        printLL((Node)ll.getFirst()) ;
        printLL(reverseLL((Node)ll.getFirst())) ;
    }
}

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