很简洁的几个答案
594 Longest Harmonious Subsequence

class Solution {
public:
    int findLHS(vector<int>& nums) {
        int res = 0;
        map<int, int> m;
        for (int num : nums) ++m[num];
        for (auto a : m) {
            if (m.count(a.first + 1)) {
                res = max(res, m[a.first] + m[a.first + 1]);
            }
        }
        return res;
    }
};

575 Distribute Candies

class Solution {  
public:  
    int distributeCandies(vector<int>& candies) {
    unordered_set<int> candyType(candies.begin(), candies.end());
    return min(candyType.size(),candies.size() / 2);
    }
};  

总结
unordered_map <类型1，类型2> hash; //哈希表数据类型
vector.push_back() //增加新元素
find(vec.begin(),vec.end(),value) != vec.end(); //判断vec中存在value元素