<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) {
        for (int step = 1; step < lists.size(); step *= 2) {
            for (int i = 0; i < lists.size(); i += step * 2) {
                if (i + step < lists.size()) {
                    lists[i] = mergeTwoLists(lists[i], lists[i + step]);
                }
            }
        }
        return lists.empty() ? nullptr : lists[0];
    }

private:
    ListNode *mergeTwoLists(ListNode *head1, ListNode *head2) {
        ListNode *dummyHead = new ListNode(-1);
        ListNode *curr = dummyHead;
        while (head1 || head2) {
            int val1 = head1 ? head1->val : INT_MAX;
            int val2 = head2 ? head2->val : INT_MAX;
            if (val1 <= val2) {
                curr->next = head1;
                head1 = head1->next;
            } else {
                curr->next = head2;
                head2 = head2->next;
            }
            curr = curr->next;
        }
        ListNode *head = dummyHead->next;
        delete dummyHead;
        return head;
    }
};

// divide and conquer
// iterative

// time: O(n log k)
// space: O(1)

18600