575. Distribute Candies

问题描述

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:

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

题目链接：

---

思路分析

给一个数组，数组中的每一个数字代表一种糖果，数组的元素个数一定是偶数，求能分给妹妹的最多种类的糖果种数。

用一个map记录一共有多少种糖果，妹妹能拿到的糖果数量是n/2，多了拿不了，就返回种类和n/2中小的那个就行了。

代码

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

    }
};

时间复杂度：$O(n)$
空间复杂度：$O(n)$

---

反思

利用bitset来代替hash table。

int distributeCandies(vector<int>& candies) {
        bitset<200001> hash;
        int count = 0;
        for (int i : candies) {
            if (!hash.test(i+100000)) {
               count++;
               hash.set(i+100000);
            }
        }
        int n = candies.size();
        return min(count, n/2);
    }

性能有很大提升。