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`