remove-duplicates-from-sorted-list-ii.cpp

// Time:  O(n)
// Space: O(1)

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution{
public:
    /**
     * @param head: The first node of linked list.
     * @return: head node
     */
    ListNode * deleteDuplicates(ListNode *head) {
        ListNode dummy{0};
        auto prev = &dummy;
        while (head) {
            if (head->next && head->next->val == head->val) {
                auto val = head->val;
                while (head && head->val == val) {
                    head = head->next;
                }
                prev->next = head;
            } else {
                prev->next = head;
                prev = head;
                head = head->next;
            }
        }
        return dummy.next;
    }
};