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:

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

Example 2:

**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

/**
 * 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;
    }
}