Question

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.

Example 1:

add(1); add(3); add(5);
find(4) -> true
find(7) -> false

Example 2:

add(3); add(1); add(2);
find(3) -> true
find(6) -> false

Solutions:

  • Method 1: HashMap + List 
      class TwoSum {
      private List<Integer> list;
      private Map<Integer, Integer> map;
      /** Initialize your data structure here. */
      public TwoSum() {
          this.list = new ArrayList<>();
          this.map = new HashMap<>();
      }
        
      /** Add the number to an internal data structure.. */
      public void add(int number) {
          this.list.add(number);
          this.map.put(number, map.getOrDefault(number, 0) + 1);
      }
        
      /** Find if there exists any pair of numbers which sum is equal to the value. */
      public boolean find(int value) {
          for(int i = 0; i < list.size(); i++){
              int num = list.get(i);
              int another = value - num;
              if(num == another && map.get(num) >= 2) return true;
              else if(num != another && map.containsKey(another)) return true;
          }
          return false;
      }
  }
    
  /**
   * Your TwoSum object will be instantiated and called as such:
   * TwoSum obj = new TwoSum();
   * obj.add(number);
   * boolean param_2 = obj.find(value);
   */