LeetCode 170. Two Sum III - Data structure design （两数之和之三 - 数据结构设计）$

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

---

题目标签：Hash Table

　　题目让我们设计一个 两数之和的 class， 要有add（number） 和 find（value）功能。

　　建立HashMap，number 当作 key；number 出现的次数当作 value保存。

　　要注意的是，两数之和的 两数可以是相同的数字，出现两次。比如 2 + 2 = 4 也是可以的。

Java Solution:

Runtime beats 82.39%

完成日期：05/16/2017

关键词：HashMap

关键点：HashMap<number, occurrence>

 class TwoSum
 {
     HashMap<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)
     {
         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 num: map.keySet())
         {
             int target = value - num;

             if(num == target)
             {
                 if(map.get(num) > 1)
                     return true;
             }
             else
             {
                 if(map.containsKey(target))
                     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);
  */

参考资料：N/A

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