# 170 Two Sum III - Data structure design

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
```

**The Idea:** Maintain a hash table of numbers. Since we are only managing pair-wise elements, map whether or not the number is been repeated. If so, this tells us that it can used twice. For example, if `value = 12` and we have `[6:true]`, we can return true. In general, we iterate through the hash table, and look the difference that is the current difference between i and value. If the difference is found, we know that i exists (because we iterated through it) and the difference also exist, which together make up the value.

**Complexity:** O(N) time for add (as the hash table grows it will occasionally have to rehash on expansion) and find. O(N) space.

```cpp
class TwoSum {
public:
    /** Initialize your data structure here. */
    TwoSum() {}

    /** Add the number to an internal data structure.. */
    void add(int number) {
        if (nums.find(number) == nums.end())
            nums.insert({number, false});
        else nums[number] = true;
    }

    /** Find if there exists any pair of numbers which sum is equal to the value. */
    bool find(int value) {
        for (auto i : nums) {
            int diff = value - i.first;
            if (diff != i.first && nums.find(diff) != nums.end()) {
                return true;
            }
            else if (diff == i.first && nums.find(diff) != nums.end() && nums[diff]) {
                return true;
            }
        }
        return false;
    }

    unordered_map<int, bool> nums;
};
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://maksimdan.gitbook.io/interview-practice-problems/leetcode_sessions/170-two-sum-iii-data-structure-design.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.