Day-10 Flatten a Multilevel Doubly Linked List

/*
// Definition for a Node.
class Node {
    public int val;
    public Node prev;
    public Node next;
    public Node child;

    public Node() {}

    public Node(int _val,Node _prev,Node _next,Node _child) {
        val = _val;
        prev = _prev;
        next = _next;
        child = _child;
    }
};
*/
class Solution {
    public Node flatten(Node head) {
        if(head==null) return head;
        Node temp=head;
        while(temp!=null){
            
            if(temp.child!=null){
                Node next=temp.next;
                Node child=flatten(temp.child);
                temp.child=null;
                temp.next=child;
                child.prev=temp;
                while(child.next!=null){
                    child=child.next;
                }
                child.next=next;
                if(next!=null)
                next.prev=child;
                temp=child;
            }
            
            temp=temp.next;
        } 
        return head;    
    }    
}

class Solution {
  public Node flatten(Node head) {
    if (head == null) return head;

    Node pseudoHead = new Node(0, null, head, null);
    Node curr, prev = pseudoHead;

    Deque<Node> stack = new ArrayDeque<>();
    stack.push(head);

    while (!stack.isEmpty()) {
      curr = stack.pop();
      prev.next = curr;
      curr.prev = prev;

      if (curr.next != null) stack.push(curr.next);
      if (curr.child != null) {
        stack.push(curr.child);
        curr.child = null;
      }
      prev = curr;
    }
    pseudoHead.next.prev = null;
    return pseudoHead.next;
  }
}