LC986. Interval List Intersections

class Solution:
    def intervalIntersection(self, A: List[List[int]], B: List[List[int]]) -> List[List[int]]:
        """
        tc O(A+B) sc O(1)
        main idea: use two pointer to track overlapping area between two intervels. once one intervel exhaust, move to next one 
        assuming both A,B list have overlaps 
        1. check corner case: one of them empty
        2. use criss-cross lock to check overlaps, update res 
        3. no matter if any overlap found, move exhausted intervel pointer to next 
        """
        n1,n2 = len(A), len(B)
        # early termination 
        if not n1 or not n2:
            return []
        p1,p2 = 0,0
        res = []
        while p1 < n1 and p2 < n2:
            As,Ae = A[p1]
            Bs,Be = B[p2]
            # there is an overlap --- Criss-cross lock
            if Bs <= Ae and As <=Be :
                res.append([max(As,Bs),min(Ae,Be)])
            if Ae <= Be: 
                p1 += 1
            else:   # Be < Ae 
                p2 += 1 
        return res