[itint5]合并K个有序链表

merge sort，leet code里面曾经做过。但一开始没这么写，遍历来做，效率n*k了，用了merge sort后，变成logn*k。

用了dummy node。同时要注意size为0的情况。

#include <climits>

/*链表结点的定义(请不要在代码中定义该类型)
struct ListNode {
  int val;
  ListNode *next;
};
*/
//lists包含k个链表的头结点,返回合并后链表头结点
ListNode* merge(vector<ListNode*> &lists) {
    if (lists.size() == 0) return NULL;
    int k = 1;
    while (k < lists.size()) {
        for (int i = 0; i < lists.size(); i += k*2) {
            int j = i + k;
            if (j >= lists.size()) break;
            // merge list i and j
            ListNode *dummy = new ListNode();
            ListNode *last = dummy;
            while (lists[i] != NULL && lists[j] != NULL) {
                if (lists[i]->val < lists[j]->val) {
                    last->next = lists[i];
                    last = last->next;
                    lists[i] = lists[i]->next;
                } else {
                    last->next = lists[j];
                    last = last->next;
                    lists[j] = lists[j]->next;
                }
            }
            while (lists[i] != NULL) {
                last->next = lists[i];
                last = last->next;
                lists[i] = lists[i]->next;
            }
            while (lists[j] != NULL) {
                last->next = lists[j];
                last = last->next;
                lists[j] = lists[j]->next;
            }
            last->next = NULL;
            lists[i] = dummy->next;
            delete dummy;
        }
        k *= 2;
    }
    return lists[0];
}