<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) {}
 * };
 */
typedef std::set<int> indices;
typedef std::map<int, indices> node_queue;
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        ListNode* head {nullptr};
        ListNode* curr {nullptr};

        int lastVal;
        int idx;
        indices working;

        node_queue vals;

        // Initialize
        for (int i {0}; i < lists.size(); ++i) {
            if (lists[i]) {
                vals[lists[i]->val].insert(i);
            }
        }

        while (!vals.empty()) {
            lastVal = vals.begin()->first;

            while (!vals[lastVal].empty()) {
                if (!head) { // Handle empty list case
                    head = new ListNode(lastVal);
                    curr = head;
                }
                else {
                    curr->next = new ListNode(lastVal);
                    curr = curr->next;
                }
                idx = *(vals[lastVal].begin());
                vals[lastVal].erase(idx);
                lists[idx] = lists[idx]->next;
                if (lists[idx]) {
                    vals[lists[idx]->val].insert(idx);
                }
            }

            vals.erase(lastVal);


        }

        return head;
        

    }
};
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