23. Merge k Sorted Lists

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

利用mergeTwoSortedLists的结果

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        if(lists.empty())   return nullptr;

        while(lists.size() >1){
            lists.push_back(mergeTwoSortedLists(lists[0], lists[1]));
            lists.erase(lists.begin());
            lists.erase(lists.begin());
        }

        return lists.front();
    }

    ListNode* mergeTwoSortedLists(ListNode *l1, ListNode *l2){
        if(l1 == nullptr)   return l2;
        if(l2 == nullptr)   return l1;

        if(l1->val < l2->val){
            l1->next = mergeTwoSortedLists(l1->next, l2);
            return l1;
        }else{
            l2->next = mergeTwoSortedLists(l1, l2->next);
            return l2;
        }
    }
};

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 struct cmp{
     bool operator() (const ListNode *a, const ListNode *b){
         return a->val > b->val;
     }
 };

class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        priority_queue<ListNode *, vector<ListNode *>, cmp>  pq;
        //注意此处要判断是否为nullptr，再加入队列中,nullptr也可以加入队列中
        for(auto &l:lists)
            if(l)
                pq.push(l);

        if(pq.empty())  return nullptr;
        ListNode *head = pq.top();
        pq.pop();

        if(head->next)  pq.push(head->next);
        ListNode *tail = head;

        while(!pq.empty()){
            tail->next = pq.top();
            pq.pop();
            tail = tail->next;
            if(tail->next) pq.push(tail->next);
        }
        return head;
    }
};

测试用例

[[1, 2, 3], [4, 5, 6],[7, 8, 9],[0, 10]]