VB.NET Array Intersection

Question

This could be terribly trivial, but I\'m having trouble finding an answer that executes in less than n^2 time. Let\'s say I have two string arrays and I want to know which s

Answer 1

Sort both lists. Then you can know with certainty that if the next entry in list A is 'cobble' and the next entry in list B is 'definite', then 'cobble' is not in list B. Simply advance the pointer/counter on whichever list has the lower ranked result and ascend the rankings.

For example:

List 1: D,B,M,A,I
List 2: I,A,P,N,D,G

sorted:

List 1: A,B,D,I,M
List 2: A,D,G,I,N,P

A vs A --> match, store A, advance both
B vs D --> B D vs D --> match, store D, advance both
I vs G --> I>G, advance 2
I vs I --> match, store I, advance both
M vs N --> M List 1 has no more items, quit.
List of matches is A,D,I

2 list sorts O(n log(n)), plus O(n) comparisons makes this O(n(log(n) + 1)).