Leetcode 82. Remove Duplicates from Sorted List II

82. Remove Duplicates from Sorted List II
    Medium
    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.

解法1：

/**
 * 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) return NULL;
        ListNode* dummy = new ListNode(0), *p1 = dummy, *p2 = head;
        dummy->next = head;
        while (p2 && p2->next) {
            if (p2->val == p2->next->val) {
                while (p2 && p2->next && p2->val == p2->next->val) {
                    p2 = p2->next;
                }
                p1->next = p2->next;
            } else {
                p1 = p1->next;    
            }
            p2 = p2->next;
        }
        return dummy->next;
    }
};