25. Reverse Nodes in k-Group

Given the head of a linked list, reverse the nodes of the list k at a time, and return the modified list.

class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode dummy = new ListNode(0);
        dummy.next=head;
        ListNode lastEnd=dummy;
        Stack<ListNode> stack=new Stack<>();
        while(head!=null){
            ListNode first=head;
            while(stack.size()<k&&head!=null){
                stack.push(head);
                head=head.next;
            }
            if(stack.size()<k) {
                lastEnd.next=first;
                break;
            }
            lastEnd.next=help(stack);
            lastEnd=first;
        }
        return dummy.next;
    }
    ListNode help(Stack<ListNode> stack){
        if(stack.size()==0) return null;
        ListNode node = stack.pop();
        node.next=help(stack);
        return node;
    }
}