[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];

}