Delete duplicate elements in the ordered linked list -ii

problem

A linked list sorted in ascending order , Delete all duplicate elements in the linked list , Only keep the elements that appear only once in the original linked list .

Ideas

Use the speed double pointer ,fast To detect non repeating elements ,slow Responsible for recording non repeating elements .

Code implementation

/** * struct ListNode { * int val; * struct ListNode *next; * }; */

class Solution {
    
public:
    /** * * @param head ListNode class  * @return ListNode class  */
    ListNode* deleteDuplicates(ListNode* head) {
    
        // write code here
        ListNode* dummyHead = new ListNode(0);
        dummyHead->next = head;
        ListNode* slow = dummyHead;
        ListNode* fast = dummyHead->next;
        while (fast && fast->next)
        {
    
            //  Next node  !=  Current node , The current node is not a repeating element 
            if (fast->next->val != fast->val)
            {
    
                // fast  What is detected is not a duplicate node , therefore  slow  You can move forward directly 
                slow = fast;
                fast = fast->next;
            }
            else
            {
    
                // fast  Duplicate elements detected , Record duplicate elements , And move forward 
                int repeatVal = fast->val;
                // fast  Not equal to repeating elements , Exit loop , There is no guarantee that this node is not a duplicate node 
                //  Such as ：1 2 2 3 3, Continue to compare  fast  And the next element 
                while (fast && fast->val == repeatVal)
                {
    
                    fast = fast->next;
                }
                
                slow->next = fast;
            }
        }
        return dummyHead->next;
    }
};