# 170. 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.

**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
```

**Thoughts:**

1. Find vs Add trade off
2. Find-favored algorithms
3. Add- favored algorithms

**Code:** algorithms that are Find-favored algorithms (**TLE**)

```java
public class TwoSum {
        Set<Integer> sum;
        Set<Integer> num;

        TwoSum(){
            sum = new HashSet<Integer>();
            num = new HashSet<Integer>();
        }
        // Add the number to an internal data structure.
        public void add(int number) {
            if(num.contains(number)){
                sum.add(number * 2);
            }else{
                Iterator<Integer> iter = num.iterator();
                while(iter.hasNext()){
                    sum.add(iter.next() + number);
                }
                num.add(number);
            }
        }

        // Find if there exists any pair of numbers which sum is equal to the value.
        public boolean find(int value) {
            return sum.contains(value);
        }
    }
```

**Code: algorithms that are Add-favored algorithms (Add: O(1), Find: (O(n))**

```java
class TwoSum {
    Map<Integer, Integer> map;
    /** Initialize your data structure here. */
    public TwoSum() {
        map = new HashMap<>();
    }

    /** Add the number to an internal data structure.. */
    public void add(int number) {
        if(map.containsKey(number)){
            map.put(number, 2);
        }
        else{
            map.put(number, 1);
        }
    }

    /** Find if there exists any pair of numbers which sum is equal to the value. */
    public boolean find(int value) {
            Iterator<Integer> iter = map.keySet().iterator();
            while(iter.hasNext()){
                int n1 = iter.next();
                int n2 = value - n1;
                if (map.containsKey(n2) &&(map.get(n2) == 2 || n1!=n2)){
                    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);
 */
```
