[LeetCode] Two Sum III - Data Structure Design

Problem Description:

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

---

Well, the basic idea is simple: just store the added numbers in an internal data structure. When we are asked to find a value, we just iterate over all the numbers in the internal data structure. For each number num, if value - num is also in the data structurue and not at the same "position" as num, then return true. After iterating over all num and we have not returnred true, return false.

The key to an efficient implementation lies in what data structure to store the added numbers. Since there may be duplicate numbers, personally I think using an unordered_map is a nice choice. It uses num as key and its number of appearances as value. It enalbes both O(1) insertion and O(1) query, and thus O(1) for add and O(n) for find if there are n elements in total.

The code is as follows.

class TwoSum {
public:
    void add(int number) {
        data[number]++;
    }

    bool find(int value) {
        for (auto pr : data) {
            int first = pr.first, second = value - first;
            if ((first != second && data.find(second) != data.end()) || (first == second && data[first] > 1))
                return true;
        }
        return false;
    }
private:
    unordered_map<int, int> data;
};