Design and implement a TwoSum class. It should support the following operations: add and find.
add - Add the number to an internal data structure.
find - Find if there exists any pair of numbers which sum is equal to the value.
Example 1:
add(1); add(3); add(5); find(4) -> true find(7) -> false
Example 2:
add(3); add(1); add(2); find(3) -> true find(6) -> false