[LeetCode] 148. Sort List

思路:
Merge Sort解决. 很简单. 但是好像因为用了递归, 所以空间开销其实是logn的…第二遍过得时候再总结纯循环版的吧…

ListNode* merge(ListNode* first, ListNode* second) {
    ListNode dummy = ListNode(0);
    ListNode* head = &dummy;
    while (first && second) {
        if (first->val < second->val) {
            head->next = first;
            first = first->next;
        }
        else {
            head->next = second;
            second = second->next;
        }
        head = head->next;
    }
    if (first)
        head->next = first;
    if (second)
        head->next = second;
    return dummy.next;
}

ListNode* sortList(ListNode* head) {
    if (! head || ! head->next) 
        return head;
    ListNode* walker = head, *runner = head;
    while (runner->next && runner->next->next) {
        walker = walker->next;
        runner = runner->next->next;
    }

    ListNode* first = head;
    ListNode* second = walker->next;
    walker->next = NULL;

    first = sortList(first);
    second = sortList(second);

    return merge(first, second);
}