Two Sum III - Data structure design(leetcode170）

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

A simpler approach is to store each input into a hash table with the input as key and its count as value. To find if a pair sum exists, just iterate through the hash table in O(n) runtime. Make sure you are able to handle duplicates correctly. 

 

public class TwoSum{
    //Firstly, we create a hashmap
    Map<Integer, Integer> hm = new HashMap<>();

    public void add(int number){
        int count = hm.containsKey(number) ? hm.get(number): 0;
        hm.put(number, count +1);
   }

   public boolean find(int value){
        //The Map.Entry interface enables you to work with a map entry
        for(Map.Entry<Integer, Integer> entry: hm.entrySet()){
            int num = entry.getKey();
            int r = value - num;
            if(r == num){
                // For duplicates, ensure there are at least two individual numbers
                if(entry.getValue() >= 2) return true;
             }else if(hm.containsKey(r)){
                return true;
            }
        }
        return false;
  }
}