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# 0082. Remove Duplicates from Sorted List II

<https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii>

## Description

Given the `head` of a sorted linked list, *delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list*. Return *the linked list **sorted** as well*.

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/01/04/linkedlist1.jpg)

```
**Input:** head = [1,2,3,3,4,4,5]
**Output:** [1,2,5]
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2021/01/04/linkedlist2.jpg)

```
**Input:** head = [1,1,1,2,3]
**Output:** [2,3]
```

**Constraints:**

* The number of nodes in the list is in the range `[0, 300]`.
* `-100 <= Node.val <= 100`
* The list is guaranteed to be **sorted** in ascending order.

## ac

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */

// Key point is: discuss 2 situation separately, duplicate or not.
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        // edge case
        if (head == null || head.next == null) return head;

        // dummy
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode h = head, t = dummy;

        // walk
        while (h != null) {
            if (h.next != null && h.next.val == h.val) {
                while (h.next != null && h.next.val == h.val) h.next = h.next.next;
                t.next = h.next;
            } else {
                t = t.next;
            }
            h = h.next;
        }

        return dummy.next;
    }
}
```