✡ 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

设计一个two sum 的类。包含add和find两种操作方式。

1、使用HashMap和HashSet实现。

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

    // Find if there exists any pair of numbers which sum is equal to the value.
    public boolean find(int value) {
        for (int num : set){
            if (num == value - num){
                if (map.get(num) == 2){
                    return true;
                }
            } else if (set.contains(value - num)){
                return true;
            }
        }
        return false;
    }
}


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

2、使用HashMap 和 List（也可以只使用一个HashMap，记录一或者2即可。）

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

    // Add the number to an internal data structure.
    public void add(int number) {
        if (map.containsKey(number)) map.put(number, map.get(number) + 1);
        else {
            map.put(number, 1);
            list.add(number);
        }
    }

    // Find if there exists any pair of numbers which sum is equal to the value.
    public boolean find(int value) {
        for (int i = 0; i < list.size(); i++){
            int num1 = list.get(i), num2 = value - num1;
            if ((num1 == num2 && map.get(num1) > 1) || (num1 != num2 && map.containsKey(num2))) return true;
        }
        return false;
    }
}


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

3、为什么要使用一个List，因为在遍历操作的时候，List要比Map或者Set快得多，使用一个List可以用空间换取时间。