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 = 1 0 <= starti < endi <= 109 endi < starti+1 0 <= startj < endj <= 109 endj < startj+1