Reorder List

• 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* next = runner->next;
        runner->next = dummy->next;
        dummy->next = runner;
        runner = next;
    }
    return dummy->next;
}

ListNode* Solution::reorderList(ListNode* A) {
    if(A->next == NULL or A->next->next == NULL) return A;
    
    ListNode* slow = A;
    ListNode* fast = A;
    while(fast->next and fast->next->next) {
        slow = slow->next;
        fast = fast->next->next;
    }
    ListNode* back = slow->next;
    slow->next = NULL;
    ListNode* rev = reverse(back);
    ListNode* dummy = new ListNode(-1);
    ListNode* runner = dummy;
    
    while(A and rev) {
        if(A) {
            runner->next = A;
            A = A->next;
            runner = runner->next;
            runner->next = NULL;
        }
        if(rev) {
            runner->next = rev;
            rev = rev->next;
            runner = runner->next;
            runner->next = NULL;
        }
    }
    if(A) {
        runner->next = A;
        A = A->next;
        runner = runner->next;
        runner->next = NULL;
    }
    
    return dummy->next;
}