algorithm - checking if 2 numbers of array add up to I

Question

I saw a interview question as follows: Give an unsorted array of integers A and and an integer I, find out if any two members of A add up to I.

any clues?

time complexity should be less

Answer 1

Insert the elements into hashtable.

While inserting x, check if I-x already exists. O(n) expected time.

Otherwise, sort the array ascending (from index 0 to n-1). Have two pointers, one at max and one at min (call them M and m respectively).

If a[M] + a[m] > I then M-- 
If a[M] + a[m] < I then m++
If a[M] + a[m] == I you have found it
If m > M, no such numbers exist.