Interval List Intersections
<p>You are given two lists of closed intervals, <code>firstList</code> and <code>secondList</code>, where <code>firstList[i] = [start<sub>i</sub>, end<sub>i</sub>]</code> and <code>secondList[j] = [start<sub>j</sub>, end<sub>j</sub>]</code>. Each list of intervals is pairwise <strong>disjoint</strong> and in <strong>sorted order</strong>.</p>

<p>Return <em>the intersection of these two interval lists</em>.</p>

<p>A <strong>closed interval</strong> <code>[a, b]</code> (with <code>a &lt;= b</code>) denotes the set of real numbers <code>x</code> with <code>a &lt;= x &lt;= b</code>.</p>

<p>The <strong>intersection</strong> of two closed intervals is a set of real numbers that are either empty or represented as a closed interval. For example, the intersection of <code>[1, 3]</code> and <code>[2, 4]</code> is <code>[2, 3]</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0900-0999/0986.Interval%20List%20Intersections/images/interval1.png" style="width: 700px; height: 194px;" />
<pre>
<strong>Input:</strong> firstList = [[0,2],[5,10],[13,23],[24,25]], secondList = [[1,5],[8,12],[15,24],[25,26]]
<strong>Output:</strong> [[1,2],[5,5],[8,10],[15,23],[24,24],[25,25]]
</pre>

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

<pre>
<strong>Input:</strong> firstList = [[1,3],[5,9]], secondList = []
<strong>Output:</strong> []
</pre>

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

<ul>
	<li><code>0 &lt;= firstList.length, secondList.length &lt;= 1000</code></li>
	<li><code>firstList.length + secondList.length &gt;= 1</code></li>
	<li><code>0 &lt;= start<sub>i</sub> &lt; end<sub>i</sub> &lt;= 10<sup>9</sup></code></li>
	<li><code>end<sub>i</sub> &lt; start<sub>i+1</sub></code></li>
	<li><code>0 &lt;= start<sub>j</sub> &lt; end<sub>j</sub> &lt;= 10<sup>9</sup> </code></li>
	<li><code>end<sub>j</sub> &lt; start<sub>j+1</sub></code></li>
</ul>
Array; Two Pointers; Line Sweep
func intervalIntersection(firstList [][]int, secondList [][]int) [][]int {
	m, n := len(firstList), len(secondList)
	var ans [][]int
	for i, j := 0, 0; i < m && j < n; {
		l := max(firstList[i][0], secondList[j][0])
		r := min(firstList[i][1], secondList[j][1])
		if l <= r {
			ans = append(ans, []int{l, r})
		}
		if firstList[i][1] < secondList[j][1] {
			i++
		} else {
			j++
		}
	}
	return ans
}