<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 {
ListNode* merge(ListNode* head1, ListNode* head2) {
        if (!head1)
            return head2;
        if (!head2)
            return head1;
        ListNode* ptr1 = head1;
        ListNode* ptr2 = head2;
        ListNode* result = NULL;
        ListNode* tail = NULL;

        while(ptr1 && ptr2) {
            if (ptr1->val < ptr2->val) {
                if (tail) {
                    tail->next = ptr1;
                    tail = tail->next;
                } else {
                    result = tail = ptr1;
                }
                ptr1 = ptr1->next;
                tail->next = NULL;
            } else {
                if (tail) {
                    tail->next = ptr2;
                    tail = tail->next;
                } else {
                    result = tail = ptr2;
                }
                ptr2 = ptr2->next;
                tail->next = NULL;
            }
        }

        if (ptr1) {
            tail->next = ptr1;
        } else {
            tail->next = ptr2;
        }
        return result;
    }
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        int n = lists.size();
        if (n == 0)
            return {};
        if (n == 1) {
            return lists[0];
        }
        int lim = log2(n);
        for (int x = 0; x <= lim; x++) {
            int step = 2 << x;
            int next = step >> 1;
            for (int i = 0; i < n; i += step) {
                if (i + next < n)
                    lists[i] = merge(lists[i], lists[i + next]);
            }
        }
        return lists[0];  
    }
};
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