LeetCode 170. Two Sum III

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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