170. Two Sum III - Data structure design

https://leetcode.com/problems/two-sum-iii-data-structure-design/description/

Time Limit Exceeded:

class TwoSum {
    private LinkedList list;
    private HashSet set;
        
    /** Initialize your data structure here. */
    public TwoSum() {
        list = new LinkedList<>();
        set = new HashSet<>();
    }
    
    /** Add the number to an internal data structure.. */
    public void add(int number) {
        for (int item: list) {
            set.add(item + number);
        }
        list.add(number);
    }
    
    /** Find if there exists any pair of numbers which sum is equal to the value. */
    public boolean find(int value) {
        return set.contains(value);
    }
}

passed: LinkedList + HashMap

这种解法有两个好处：

• 易于重复性很多的数值
• Extension： 如果需要进行删除操作， 这种写法更容易。

个人更喜欢上面 HashSet 的解法，find 操作 O(1)。看哪个操作使用的更频繁了。

class TwoSum {
    private LinkedList list;   // uniq number
    private HashMap map; // item and its count
        
    /** Initialize your data structure here. */
    public TwoSum() {
        list = new LinkedList<>();
        map = new HashMap<>();
    }
    
    /** Add the number to an internal data structure.. */
    public void add(int num) {
        if (map.containsKey(num)) {
            map.put(num, map.get(num) + 1);
        } else {
            map.put(num, 1);
            list.add(num);
        }        
    }
    
    /** Find if there exists any pair of numbers which sum is equal to the value. */
    public boolean find(int value) {
        for (Integer num1: list) {
            int num2 = value - num1;
            if ((num1 == num2 && map.get(num1) >= 2) || (num1 != num2) && map.containsKey(num2)) {
                return true;
            } 
        }
        return false;
    }
}