Description

Submission

With fake head and nested iterative deletion, this should be faster. But I’ll just skip it.

/**
 * 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;
        bool removeHead = false;
        if(head->val == head->next->val) removeHead = true;
        
        unordered_map<int, int> counter;

        for(ListNode* ptr = head; ptr != nullptr; ptr = ptr->next) {
            counter[ptr->val]++;
        }
        
        for(ListNode *ptr = head->next, *prevNode = head;
            ptr != nullptr; ptr = ptr->next) {
            auto iter = counter.find(ptr->val);
            if(iter->second > 1) {
                prevNode->next = ptr->next;
            } else {
                prevNode = ptr;
            }
        }
        
        // handle the head
        if(removeHead) {
            head = head->next;
        }
        return head;
    }
};