You are given two lists of closed intervals, firstList and secondList , where firstList[i] = [start _i , end _i ] and secondList[j] = [start _j , end _j ] . 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] .
0 <= firstList.length, secondList.length <= 1000firstList.length + secondList.length >= 10 <= start _i < end _i <= 10 ^9end _i < start _i+10 <= start _j < end _j <= 10 ^9end _j < start _j+1