<p>You are given an array of <code>k</code> linked-lists <code>lists</code>, each linked-list is sorted in ascending order.</p>

<p><em>Merge all the linked-lists into one sorted linked-list and return it.</em></p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> lists = [[1,4,5],[1,3,4],[2,6]]
<strong>Output:</strong> [1,1,2,3,4,4,5,6]
<strong>Explanation:</strong> The linked-lists are:
[
  1-&gt;4-&gt;5,
  1-&gt;3-&gt;4,
  2-&gt;6
]
merging them into one sorted list:
1-&gt;1-&gt;2-&gt;3-&gt;4-&gt;4-&gt;5-&gt;6
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> lists = []
<strong>Output:</strong> []
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> lists = [[]]
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>k == lists.length</code></li>
	<li><code>0 &lt;= k &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= lists[i].length &lt;= 500</code></li>
	<li><code>-10<sup>4</sup> &lt;= lists[i][j] &lt;= 10<sup>4</sup></code></li>
	<li><code>lists[i]</code> is sorted in <strong>ascending order</strong>.</li>
	<li>The sum of <code>lists[i].length</code> will not exceed <code>10<sup>4</sup></code>.</li>
</ul>

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class ListNodeComparator {
    public:
       bool operator()(ListNode* left, ListNode* right){
           if(left->val >= right->val){
               return true;
           }
           return false;
      }
};

class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        priority_queue<ListNode*, vector<ListNode*>, ListNodeComparator> pq;

        for(int i = 0; i < lists.size(); i++) {
            auto it = lists[i];
            while(it != nullptr) {
                 pq.push(it);
                 it = it->next;
            }
        }
        cout<<pq.size()<<endl;
        ListNode* root = nullptr;
        ListNode* it = root;
        while(!pq.empty())
        {
            auto node = pq.top();
            pq.pop();
            if(root == nullptr) {
                root = new ListNode(node->val);
                it = root; 
            }
            else if(node != nullptr)
            {
                it->next = new ListNode(node->val);
                it = it->next;
            }
            cout<<node->val<<" "<<it->val<<endl;
        }

        return root;
        //ListNode* root = new ListNode(1);
        // pq.pop();
        // auto it = root;
        // while(!pq.empty()) {
        //     it->next = new ListNode(pq.top());
        //     pq.pop();
        //     it = it->next;
        // }

        return nullptr;
    }
};
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