05. Remove Duplicates from Sorted List II.cpp

/*
Remove Duplicates from Sorted List II
=====================================

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.
*/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */

class Solution
{
public:
  ListNode *deleteDuplicates(ListNode *head)
  {
    if (!head || !head->next)
      return head;
    if (!head->next->next)
    {
      if (head->val == head->next->val)
        return NULL;
      else
        return head;
    }

    ListNode *dummy = new ListNode(0);
    dummy->next = head;
    auto pre = dummy, curr = head;

    while (curr)
    {
      while (curr->next && curr->val == curr->next->val)
        curr = curr->next;
      if (pre->next == curr)
        pre = pre->next;
      else
        pre->next = curr->next;
      curr = curr->next;
    }

    return dummy->next;
  }
};