Two Sum III - Data structure design

Source

• leetcode: 170

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

java

public class TwoSum {
    HashMap<Integer, Integer> map;
    public TwoSum() {
        map = new HashMap<Integer, Integer>();
    }

    public void add(int x) {
        if (map.containsKey(x)) {
            map.put(x, map.get(x)+1);
        }
        else {
            map.put(x, 1);
        }
    }

    public boolean find(int target) {
        for (int i : map.keySet()) {
            if (map.containsKey(target-i)) {
                if (target - i != i) return true;
                else if (map.get(i) >= 2) return true;
            }
        }
        return false;
    }
}