Remove Duplicates from Sorted List @LeetCode

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3.

	package leetcode.linkedlist;
	import java.util.Hashtable;
	import javax.swing.text.html.HTMLDocument.RunElement;
	/**
	* Solution: One pointer - Solution 3, Two pointer(with runner) - Solution 1 (same to 3, only use runner replace current.next)
	*
	* @author jeffwan
	* @date Mar 7, 2014
	*/
	public class DeleteDuplicates {
	/**
	* Solution1:Two pointer, Iterative way. two while loops. one current, one runner. still O(n)
	* Remember to return head but not current, current already go to last node.
	* @param head
	* @return
	*/
	public ListNode deleteDuplicates(ListNode head) {
	if (head == null) {
	return null;
	}
	ListNode current = head;
	ListNode runner = head.next;
	while(runner != null) {
	if (runner.val == current.val) {
	current.next = runner.next;
	} else {
	current = runner;
	}
	runner = runner.next;
	}
	return head;
	}
	/**
	* Solution2: Single Pointer
	* This way is almost same to remove duplicates from sorted List II(with dummy node).
	* Only difference is we start from current, use current.next = current.next.next to skip current.next
	*
	* Actually, for this problem, solution1 seems more straightforward.
	*/
	public ListNode deleteDuplicates2(ListNode head) {
	if (head == null || head.next == null) {
	return head;
	}
	ListNode current = head;
	while (current!= null && current.next != null) {
	if(current.val == current.next.val) {
	int val = current.val;
	while(current.next != null && current.next.val == val) {
	current.next = current.next.next;
	}
	}
	current = current.next;
	}
	return head;
	}
	/**
	* Solution 3: Single Pointer -- Best solution I think.
	* Actually, we could use this ways directly but not solution2. Because we don't need to remember to target value.
	* Solution 4 is useless, we just need to campare with head.val, no need to use a map.
	* @param head
	* @return
	*/
	public ListNode deleteDuplicates3(ListNode head) {
	if (head == null || head.next == null) {
	return head;
	}
	ListNode newHead = head;
	while (head.next != null) {
	if (head.next.val == head.val) {
	head.next = head.next.next;
	} else {
	head = head.next;
	}
	}
	return newHead;
	}
	/**
	* Solution4: Extra Space - Hashtable -- complier error in LeetCode Online Judge but really works.
	* @param head
	* @return
	*/
	public ListNode deleteDuplicates4(ListNode head) {
	if(head == null) {
	return null;
	}
	ListNode current = head;
	Hashtable table = new Hashtable();
	table.put(current.val, true);
	while (current.next != null) {
	if (table.containsKey(current.next)) {
	current.next = current.next.next;
	} else {
	table.put(current.next.val, true);
	current = current.next;
	}
	}
	return current;
	}
	class ListNode {
	int val;
	ListNode next;
	ListNode (int x) {
	this.val = x;
	}
	}
	}

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