# LeetCode 170. Two Sum III – Data structure design

## Description

https://leetcode.com/problems/two-sum-iii-data-structure-design/

Design a data structure that accepts a stream of integers and checks if it has a pair of integers that sum up to a particular value.

Implement the `TwoSum` class:

• `TwoSum()` Initializes the `TwoSum` object, with an empty array initially.
• `void add(int number)` Adds `number` to the data structure.
• `boolean find(int value)` Returns `true` if there exists any pair of numbers whose sum is equal to `value`, otherwise, it returns `false`.

Example 1:

```Input
[[], , , , , ]
Output
```

[null, null, null, null, true, false]

Explanation TwoSum twoSum = new TwoSum(); twoSum.add(1); // [] –>  twoSum.add(3); //  –> [1,3] twoSum.add(5); // [1,3] –> [1,3,5] twoSum.find(4); // 1 + 3 = 4, return true twoSum.find(7); // No two integers sum up to 7, return false

Constraints:

• `-105 <= number <= 105`
• `-231 <= value <= 231 - 1`
• At most `5 * 104` calls will be made to `add` and `find`.

## Explanation

Use a list to track the elements added and a set to track complements.

## Python Solution

``````class TwoSum:

def __init__(self):
"""
"""
self.nums = []

def add(self, number: int) -> None:
"""
Add the number to an internal data structure..
"""
self.nums.append(number)

def find(self, value: int) -> bool:
"""
Find if there exists any pair of numbers which sum is equal to the value.
"""
complements = set()
for num in self.nums:
if value - num in complements:
return True