LeetCode - Two Sum III - Data structure design

https://leetcode.com/problems/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

这道题一开始我还想用arraylist或linkedlist来存数字，后来看别人的发现确实只需要一个hashmap就可以了哈。

这样存进去的数字只需要更新在hashmap中的次数，find的时候也只需要在hashmap中找就行了。

代码如下：

public class TwoSum {  
    private 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 num : map.keySet()){  
            int target = value - num;  
            if(target == num){  
                if(map.get(num)>=2) return true;  
            }   
            else if(map.containsKey(target)) return true;  
        }  
        return false;  
    }  
}