170. Two Sum III

题目：

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

链接： http://leetcode.com/problems/two-sum-iii-data-structure-design/

题解：

设计two sum，这道题用hashmap很适合，加入是O(1)，查找的话要遍历整个keyset，所以是O(n)。

Time Complexity - O(n) , Space Complexity - O(n)

public class TwoSum {
    Map<Integer, Integer> map = new HashMap<>();

    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(int i : map.keySet()) {
            int res = value - i;
            if(map.containsKey(res)) {
                if(res == i && map.get(i) >= 2 )
                    return true;
                if(res != i)
                    return true;
            }
        }
        
        return false;
    }
}

二刷:

使用了与一刷一样的方法，这样看起来比较慢，要改进。 

好好研究了一下discuss，发现了几个问题

1. 遍历整个map的时候，用entrySet比keySet快
2. 可以建一个set保存之前查询过并且找到答案的value
3. 使用ArrayList来保存之前加入过的数字，再遍历这个ArrayList比遍历HashMap的keySet和entrySet都要快很多...

Java:

Time Complexity - O(n) , Space Complexity - O(n)

public class TwoSum {
    Map<Integer, Integer> map = new HashMap<>();
    Set<Integer> valueSet;
    public TwoSum() {
        map = new HashMap<>();
        valueSet = new HashSet<>();
    }
    
    // Add the number to an internal data structure.
    public void add(int number) {
        if (!map.containsKey(number)) {
            map.put(number, 1);    
        } else {
            map.put(number, 2);
        }
    }

    // Find if there exists any pair of numbers which sum is equal to the value.
    public boolean find(int value) {
        if (valueSet.contains(value)) {
            return true;
        }
        for (int i : map.keySet()) {
            int remain = value - i;
            if (map.containsKey(remain)) {
                if ((remain == i && map.get(remain) > 1) || remain != i) {
                    valueSet.add(value);
                    return true;    
                } 
            }
        }
        return false;
    }
}


// Your TwoSum object will be instantiated and called as such:
// TwoSum twoSum = new TwoSum();
// twoSum.add(number);
// twoSum.find(value);

Reference:

https://leetcode.com/discuss/76823/beats-100%25-java-code