# 170 Two Sum III – Data structure design – Easy

### Problem:

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.

### Thoughts:

There could be many ways to design the data structure. With considering about performance, I choose using a HashMap, which will end up with O(1) for adding and O(n) for finding.

### Solutions:

``````public class TwoSum {
private HashMap<Integer, Integer> count = new HashMap<Integer, Integer>();
// Add the number to an internal data structure.
if (count.containsKey(number)) {
count.put(number, count.get(number) + 1);
}
else {
count.put(number, 1);
}
}

// Find if there exists any pair of numbers which sum is equal to the value.
public boolean find(int value) {
for (Integer key:count.keySet()) {
if (2*key == value && count.get(key) >=2) {
return true;
}
if (2*key != value && count.containsKey(value - key)) {
return true;
}
}
return false;
}
}

// Your TwoSum object will be instantiated and called as such:
// TwoSum twoSum = new TwoSum();