public class LinkedListMain {

    static class ListNode {
        int val;
        ListNode next;
        ListNode(int val) {
            this.val = val;
            this.next = null;
        }

        ListNode() {
            new ListNode(0);
        }

        @Override
        public String toString() {
            return printLinkedList(this);
        }
    }

    public static ListNode reverseList(ListNode head) {
        if(head == null)
            return null;

        ListNode curr = head;
        ListNode prev = null;
        while(curr != null) {
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev  =curr;
            curr = nextTemp;
        }
        return prev;
    }

    public static void main(String[] args) {
        ListNode head = convertArrayToLinkedList(new int[] {1,2,3,4,5});
        System.out.println(printLinkedList(head));
        head = reverseList(head);
        System.out.println(printLinkedList(head));
    }

    private static String printLinkedList(ListNode head) {
        StringBuilder sb = new StringBuilder();
        while(head != null) {
            sb.append(head.val + ", ");
            head = head.next;
        }
        return sb.substring(0, sb.length()-2);
    }

    private static ListNode convertArrayToLinkedList(int[] input) {
        ListNode head = new ListNode();
        ListNode dummy = head;
        for(int num: input) {
            ListNode temp = new ListNode(num);
            head.next = temp;
            head = head.next;
        }
        return dummy.next;
    }

}