leetcode170- 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

这题更考察怎么设计的问题。

数据结构用一个map不要用list。这样加是O(1)的时间，查是O(n)。map当宏观变量是能避免每次查的时候重新开一个hashset的那个空间浪费，既然做成API了就用一个整体的来存储下来，不要每次反复重新开。因为肯定是想要经常调用其中这个函数的。还有小心变量写private的细节。

class TwoSum {

    private Map<Integer, Integer> map;
    
    /** Initialize your data structure here. */
    public TwoSum() {
        this.map = new HashMap<>();    
    }
    
    /** 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);
        }
    }
    
    /** Find if there exists any pair of numbers which sum is equal to the value. */
    public boolean find(int value) {
        for (int num1 : map.keySet()) {
            int num2 = value - num1;
            if (num1 != num2 && map.containsKey(num2) || num1 == num2 && map.get(num1) > 1) {
                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);
 */