題目：
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.

思路：

代碼：

class TwoSum {
public:
    /*
     * @param number: An integer
     * @return: nothing
     */
    multiset<int> nums;
    
    void add(int number) {
        // write your code here
        nums.insert(number);    
    }

    /*
     * @param value: An integer
     * @return: Find if there exists any pair of numbers which sum is equal to the value.
     */
    bool find(int value) {
        // write your code here
        for (int i : nums) {
            int need_count;
            if (i == value-i)
                need_count = 2;
            else
                need_count = 1;
            //如果在裡面找到配對的
            if (nums.count(value - i) >= need_count) {
                return true;
            }
        }
        return false;
    }
};