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

首先想到用doubly linkedlist 来做， 每次加入的时候维护list 是sorted的， 然后查询的时候左右夹比就行了。 但是这样time out。
所以用hashmap来做。

 public class TwoSum {
     HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
     public void add(int number) {
         if(map.containsKey(number)) map.put(number, map.get(number) + 1);
         else map.put(number, 1);
     }

     public boolean find(int value) {
         for(Integer val : map.keySet()){
             if(value == val + val && map.get(val) > 1) return true;
             if(value != val + val && map.containsKey(value - val)) return true;
         }
         return false;
     }
 }