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class Solution {
public:
    ListNode* oddEvenList(ListNode* head) {
        auto evenHead = new ListNode(-1);
        auto oddHead = new ListNode(-1);

        auto evenTail = evenHead, oddTail = oddHead;

        int k = 1;
        while (head) {
            if (k & 0x1) { //奇
                evenTail->next = head;
                evenTail = head;
            } else {
                oddTail->next = head;
                oddTail = head;
            }
            k++;
            head = head->next;
        }
        
        oddTail->next = NULL; //最后的尾巴需要设置为空
        evenTail->next = oddHead->next;
        return evenHead->next;
    }
};

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplication(ListNode* head) {
        auto dummy = new ListNode(-1);
        dummy->next = head;
        auto p = dummy;
        while (p->next) {
            auto q = p->next;
            while (q->next && q->next->val == q->val) q = q->next;
            if (p->next == q) p = q;
            else {
                p->next = q->next; 
            }
        }
        
        return dummy->next;
    }
};

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* removeDuplicateNodes(ListNode* head) {
        //hash + 链表
        unordered_map<int, bool> map;

        auto dummy = new ListNode(-1);
        dummy->next = head;

        auto p = head, pre = dummy;
        while (p) {
            if (map[p->val]) { //出现过
                pre->next = p->next;
            }
            else {
                pre = p;
            }
            map[p->val] = true;
            p = p->next;
        }

        return dummy->next;
    }
};