剑指offer-问题16

package offer;

/**
 * offer interview 16
 */
public class Test16 {

    public static class ListNode{
        int value;
        ListNode next;

        public ListNode(){
        }

        public ListNode(int value){
            this.value = value;
            this.next = null;
        }

    }


    //method 1
    public static ListNode reverseList(ListNode head){
        ListNode root = new ListNode();
        root.next = null;

        ListNode next;

        while (head != null){
            next = head.next;

            head.next = root.next;
            root.next = head;

            head = next;
        }

        return root.next;

    }

    //method 2
    public static ListNode reverseList2(ListNode head){
        ListNode reverseHead = null;
        ListNode curr = head;
        ListNode prev = null;
        ListNode next;

        while (curr != null){
            reverseHead = curr;
            next = curr.next;

            curr.next = prev;
            prev = curr;
            curr = next;
        }

        return reverseHead;
    }


    public static void printList(ListNode head){
        while (head != null){
            System.out.print(head.value + "->");
            head =  head.next;
        }
        System.out.println("null");
    }

    public static void main(String[] args){
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);
        head.next.next.next.next = new ListNode(5);
        head.next.next.next.next.next = new ListNode(6);
        head.next.next.next.next.next.next = new ListNode(7);
        head.next.next.next.next.next.next.next = new ListNode(8);
        head.next.next.next.next.next.next.next.next = new ListNode(9);

        printList(head);
        head = reverseList(head);
        printList(head);

        System.out.println("========");
        head = reverseList2(head);
        printList(head);
    }

}