1<=>2<=>3<=>4 ==> 2<=>3<=>4
head
.
1. Traverse to the second node of the list, if the list has more than two nodes. If it has less than two nodes, simply return null.
2. Set its previous
reference to null
.
3. Return this new node as the head node.
public DoublyLinkedNode deleteAtHead(DoublyLinkedNode head) { }
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