合并 k 个排序链表，返回合并后的排序链表。请分析和描述算法的复杂度。
示例:
输入:
[
1->4->5,
1->3->4,
2->6
]
输出: 1->1->2->3->4->4->5->6
代码
class Solution {
public:
ListNode *mergeKLists(vector<ListNode *> &lists) {
if (lists.size() == 0) return NULL;
int n = lists.size();
while (n > 1) {
int k = (n + 1) / 2;
for (int i = 0; i < n / 2; ++i) {
lists[i] = mergeTwoLists(lists[i], lists[i + k]);
}
n = k;
}
return lists[0];
}
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
ListNode *head = new ListNode(-1);
ListNode *cur = head;
while (l1 && l2) {
if (l1->val < l2->val) {
cur->next = l1;
l1 = l1->next;
} else {
cur->next = l2;
l2 = l2->next;
}
cur = cur->next;
}
if (l1) cur->next = l1;
if (l2) cur->next = l2;
return head->next;
}
};
