575. Distribute Candies

Given an integer array with even length, where different numbers in this array represent different kinds of candies. Each number means one candy of the corresponding kind. You need to distribute these candies equally in number to brother and sister. Return the maximum number of kinds of candies the sister could gain.
Example 1:

Input: candies = [1,1,2,2,3,3]
Output: 3
Explanation:
There are three different kinds of candies (1, 2 and 3), and two candies for each kind.
Optimal distribution: The sister has candies [1,2,3] and the brother has candies [1,2,3], too. 
The sister has three different kinds of candies. 

Example 2:

Input: candies = [1,1,2,3]
Output: 2
Explanation: For example, the sister has candies [2,3] and the brother has candies [1,1]. 
The sister has two different kinds of candies, the brother has only one kind of candies. 

Note:

The length of the given array is in range [2, 10,000], and will be even.
The number in given array is in range [-100,000, 100,000].

思路:举例推演即可得以下结论:
假设数组长度为n,不同数字种类数为k,
1.若k < n/2 ,则sister最多拿到k种数.
2.若k > n/2,则由于sister和brother要相等的限制,sister最多也只能拿到n/2.
结论:sister最多拿到min(k, n/2)种.

用集合set即可统计不同数字的种类,考虑到无序集合unordered_set效率更高,速度更快.

class Solution {
public:
    int distributeCandies(vector<int>& candies) {
        unordered_set<int> hash;
        for (int i = 0; i < candies.size(); i++) {
            hash.insert(candies[i]);
        }
        return min(hash.size(), candies.size()/2);
    }
};