Given A and B two interval lists, A has no overlap inside A and B has no overlap inside B. Write the function to merge two interval lists, output the result with no overlap. Ask for a very efficient solution

A naive method can combine the two list, and sort and apply merge interval in the leetcode, but is not efficient enough.

For example,
A: [1,5], [10,14], [16,18]
B: [2,6], [8,10], [11,20]

output [1,6], [8, 20]