I used C++ to solve this problem.

Time compexity: O(n)
Space complexity: O(n), where n is the number of elements of the array.

Functionality:
The program receives a file "1.in" with the following information: on the first
line the number M and the size of the array and on the next line, the elements
of the array and it generates an output file "1.out" with all the pairs.
format: a-b c-d e-f...

Description:
I traverse the array only once. I am using an unordered set/hash map
because its functions have a compexity of O(1) which does not affect the overall
compexity.
I use a hash map to look for the current value int he array, if it is a value
which after substracting the former one we obtain the value M, I print it.