Even Reverse

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
ListNode* reverse(ListNode* A) {
    ListNode* dummy = new ListNode(-1);
    ListNode* runner = A;
    while(runner) {
        ListNode* nxt = runner->next;
        runner->next = dummy->next;
        dummy->next = runner;
        runner = nxt;
    }
    
    return dummy->next;
}
ListNode* Solution::solve(ListNode* A) {
    bool fl = true;
    ListNode* odd = new ListNode(-1);
    ListNode* even = new ListNode(-1);
    ListNode* otail = odd, *etail = even;
    ListNode* runner = A;
    while(runner) {
        if(fl) {
            otail->next = runner;
            otail = otail->next;
        } else {
            etail->next = runner;
            etail = etail->next;
        }
        runner = runner->next;
        otail->next = etail->next = NULL;
        fl = !fl;
    }
    ListNode* rev = reverse(even->next);
    ListNode* res = new ListNode(-1);
    runner = res;
    odd = odd->next;
    fl = true;
    while(odd or rev) {
        if(fl) {
            runner->next = odd;
            odd = odd->next;
        } else {
            runner->next = rev;
            rev = rev->next;
        }
        runner = runner->next;
        fl = !fl;
    }
    return res->next;
}