<p>You have <code>k</code> lists of sorted integers in <strong>non-decreasing&nbsp;order</strong>. Find the <b>smallest</b> range that includes at least one number from each of the <code>k</code> lists.</p>

<p>We define the range <code>[a, b]</code> is smaller than range <code>[c, d]</code> if <code>b - a &lt; d - c</code> <strong>or</strong> <code>a &lt; c</code> if <code>b - a == d - c</code>.</p>

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

<pre>
<strong>Input:</strong> nums = [[4,10,15,24,26],[0,9,12,20],[5,18,22,30]]
<strong>Output:</strong> [20,24]
<strong>Explanation: </strong>
List 1: [4, 10, 15, 24,26], 24 is in range [20,24].
List 2: [0, 9, 12, 20], 20 is in range [20,24].
List 3: [5, 18, 22, 30], 22 is in range [20,24].
</pre>

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

<pre>
<strong>Input:</strong> nums = [[1,2,3],[1,2,3],[1,2,3]]
<strong>Output:</strong> [1,1]
</pre>

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

<ul>
	<li><code>nums.length == k</code></li>
	<li><code>1 &lt;= k &lt;= 3500</code></li>
	<li><code>1 &lt;= nums[i].length &lt;= 50</code></li>
	<li><code>-10<sup>5</sup> &lt;= nums[i][j] &lt;= 10<sup>5</sup></code></li>
	<li><code>nums[i]</code>&nbsp;is sorted in <strong>non-decreasing</strong> order.</li>
</ul>

struct entry {
    int val;
    int listInd;
    int ind;
};

inline bool operator<(const entry& lhs, const entry& rhs)
{
    return tie(lhs.val, lhs.listInd, lhs.ind) < tie(rhs.val, rhs.listInd, rhs.ind);
}

class Solution {
public:
    vector<int> smallestRange(vector<vector<int>>& nums) {
        set<entry> s;

        for (int i = 0; i < nums.size(); i++) {
            entry e(nums[i][0], i, 0);
            s.insert(e);
        }

        vector<int> bestRange;
        int bestLength = 201 * 1000;

        while (true) {
            int curRangeSt = s.begin()->val;
            int curRangeEnd = prev(s.end())->val;
            int curRangeLength = curRangeEnd - curRangeSt;

            if (curRangeLength < bestLength) {
                bestLength = curRangeLength;
                bestRange = {curRangeSt, curRangeEnd};
            }

            auto minEntry = s.begin();
            if (minEntry->ind == nums[minEntry->listInd].size() - 1) return bestRange;


            entry e(nums[minEntry->listInd][minEntry->ind + 1], minEntry->listInd, minEntry->ind + 1);
            s.insert(e);

            s.erase(minEntry);

        }

        return bestRange;
    }
};
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