Reverse Linked List II Java

 Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.

Have you been asked this question in an interview?

Two Approach:

     Goal: reverse LinkNodes in range between m & n;

    Solution_1:
    Idea: easy to Implement if you fully understand the problem of Reverse Linked List
    This is just a extension.
    Use 3 addition nodes:
    Prev: point to position of m-1
    begin: point to position of m
    end: point to position of n
    call reverse function from Reverse Linked List by pass
    reverse(prev, end.next);

 public ListNode reverseBetween(ListNode head, int m, int n) {
        if(head==null | head.next==null) return head;
        ListNode dummy=new ListNode(0);
        dummy.next=head;
        ListNode prev=dummy;
        ListNode begin=head;
       while(m>1){
             prev=begin;
            begin=begin.next;
            m--;
        }
        ListNode end=head;
        while(n>1){
            end=end.next;
            n--;
        }
        reverse(prev,end.next);

        return dummy.next;
    }
    private ListNode reverse(ListNode dummy,ListNode afterend){
        ListNode prev=dummy;
        ListNode cur=dummy.next;
        ListNode nextNode=cur.next;
        while(nextNode!=afterend){
            cur.next=nextNode.next;
            nextNode.next=prev.next;
            prev.next=nextNode;
            nextNode=cur.next;
        }
        return dummy.next;
    }

Solution_2:
Key to solve:
Find 4 additional node as well, prevDummy, nextDummy, beign and end
prevDummy: connect with begin
beign: connect with end
nextDummy: store connection for end.next
end: connect with begin

In case of Given 1->2->3->4->5->NULL, m = 2 and n = 4,
beign node: 2; end node 4;
then reverse node using iterating methond of while loop
loop 1:  1->3->4->2->5->NULL
loop 2:  1->4->3->2->5->NULL
until beign==end, excit loop

 public ListNode reverseBetween(ListNode head, int m, int n) {
            if(head==null || head.next==null) return head;

            ListNode prevDummy=new ListNode(0);
            prevDummy.next=head;
            ListNode nextDummy;

            ListNode begin=head;
            for (int i=1;i<m;i++) {
                prevDummy=begin;
                begin=begin.next;
            }
            ListNode end=head;
            for(int j=1;j<n;j++){
                end=end.next;
            }

            while(begin!=end){
                //save end.next into nextDummy node
                nextDummy=end.next;
                //move reverse node after end node
                end.next=begin;
                //connect to next node
                prevDummy.next=begin.next;
                //connect to right most portion
                begin.next=nextDummy;
                //connect to left most portion
                begin=prevDummy.next;

            }

            //becareful: in case of reverse from begin to n =>m==1
            if(m==1) return prevDummy.next;
            else return head;

        }