Flatten a Multilevel Doubly Linked List LeetCode solution in Java

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class Solution {
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  public Node flatten(Node head) {
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    Node curr = head; // for traversal
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    Node tail = head; // for keeping the track of previous node
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    Stack<Node> stack =
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        new Stack<>(); // for storing the reference of next node when child node encounters
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    while (curr != null) {
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      if (curr.child != null) { // if there is a child
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        Node child = curr.child; // creating a node for child
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        if (curr.next != null) { // if there is list after we find child a child
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          stack.push(curr.next); // pushing the list to the stack
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          curr.next.prev = null; // pointing its previous to null
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        }
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        curr.next = child; // pointing the current's reference to child
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        child.prev = curr; // pointing child's previous reference to current.
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        curr.child = null; // pointing the current's child pointer to null
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      }
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      tail = curr; // for keeping track of previous nodes
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      curr = curr.next; // traversing
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    }
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    while (!stack.isEmpty()) { // checking if the stack has still nodes in it.
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      curr = stack.pop(); // getting the last node of the list pushed into the stack
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      tail.next = curr; // pointing the previos node to the last node
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      curr.prev = tail; // pointing previos pointer of the last node to the previos node.
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      while (curr != null) { // traversing the last node's popped out of stack
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        tail = curr;
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        curr = curr.next;
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      }
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    }
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    return head;
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  }
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}