eliminate the duplicates in a sorted linked list (1)

Qustion:

Given a sorted linked list, and remove all the nodes which have duplication. For example, 1 -> 1 -> 2 -> 2 -> 3 ->4 , after removing duplicates, the linked list becomes 3 ->4.

Analyze:

1. In order to remove the duplicated nodes in the list, we consider the duplicates as a composite node. After removing the composite node, we need to make sure that  remaining nodes are still connected. Therefore, we need to have a pointer which points to the parent node of the composite node. When the composite node is removed, the parent node will point to the next non-duplicated node.

2. If the first node belongs to a composite node, we need to return a new head.

Code:

public ListNode removeDuplicates(ListNode head) {
	if (head == null || head.next == null) return head;
	ListNode newHead = new ListNode(-1);
	ListNode preNode = newHead;
	while(head != null) {
		if (head.next != null && head.val == head.next.val) {
			while (head.next != null && head.val == head.next.val) {
				head = head.next;
			}
			head = head.next;
		} else {
			preNode.next = head;
			head = head.next;
			preNode = preNode.next;
			preNode.next = null;
		}
	}
	return newHead.next;
}

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