Interval List Intersections

You are given two lists of closed intervals, `firstList` and `secondList`, where `firstList[i] = [starti, endi]` and `secondList[j] = [startj, endj]`. Each list of intervals is pairwise **disjoint** and in **sorted order**. Return _the intersection of these two interval lists_. A **closed interval** `[a, b]` (with `a <= b`) denotes the set of real numbers `x` with `a <= x <= b`. The **intersection** 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 `[1, 3]` and `[2, 4]` is `[2, 3]`.   **Example 1:** **Input:** firstList = [[0,2],[5,10],[13,23],[24,25]], secondList = [[1,5],[8,12],[15,24],[25,26]] **Output:** [[1,2],[5,5],[8,10],[15,23],[24,24],[25,25]] ``` **Example 2:** **Input:** firstList = [[1,3],[5,9]], secondList = [] **Output:** [] ```   **Constraints:** - `0 <= firstList.length, secondList.length <= 1000` - `firstList.length + secondList.length >= 1` - `0 <= starti < endi <= 109` - `endi < starti+1` - `0 <= startj < endj <= 109 ` - `endj < startj+1`