Smallest Range Covering Elements from K Lists
<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>
Greedy; Array; Hash Table; Sorting; Sliding Window; Heap (Priority Queue)
func smallestRange(nums [][]int) []int {
	t := [][]int{}
	for i, x := range nums {
		for _, v := range x {
			t = append(t, []int{v, i})
		}
	}
	sort.Slice(t, func(i, j int) bool { return t[i][0] < t[j][0] })
	ans := []int{-1000000, 1000000}
	j := 0
	cnt := map[int]int{}
	for _, x := range t {
		b, v := x[0], x[1]
		cnt[v]++
		for len(cnt) == len(nums) {
			a, w := t[j][0], t[j][1]
			x := b - a - (ans[1] - ans[0])
			if x < 0 || (x == 0 && a < ans[0]) {
				ans[0], ans[1] = a, b
			}
			cnt[w]--
			if cnt[w] == 0 {
				delete(cnt, w)
			}
			j++
		}
	}
	return ans
}