<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 Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        ListNode* new_list = new ListNode();
        ListNode* head = new_list;
        mergeKListsRec(lists,head);
        head = head->next;
        return head;
    }
private:
    ListNode* mergeKListsRec(vector<ListNode*>& lists,ListNode* head) {

        vector<int> indices_to_erase;
        for (int i=0; i < lists.size(); ++i) {
            if (lists[i] == nullptr) {
                indices_to_erase.push_back(i);
            }
        }

        lists.erase( remove (lists.begin(), lists.end(), nullptr), lists.end() );
        

        int smallest_index = -1;
        int smallest_val = numeric_limits<int>::max();
        for (int i=0; i < lists.size(); ++i) {
            if (lists[i]->val <= smallest_val) {
                smallest_val = lists[i]->val;
                smallest_index = i;
            }
        }
        if (lists.empty()) {
            return head;
        }

        head->next = lists[smallest_index];
        lists[smallest_index] = lists[smallest_index]->next;
        head = head->next;

        return mergeKListsRec(lists, head);

    }
};
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