LeetCode Q170: Two Sum III - Data structure desing

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.

For example,

add(1); add(3); add(5);
find(4) -> true
find(7) -> false

Solution:
O(1) add
O(n) find: using hash table.

Need to care about when two numbers are equal. In this case, this number has to be added at least twice.

By using unordered_map::count() is much faster than unordered_map::find().