LeetCode 692
自定义结构体也可以在结构体内重载 <

class Solution {
public:

    typedef pair<int, string> PIS;

    struct comp {
        bool operator() (PIS& a, PIS& b) {
            if (a.first != b.first) return a.first < b.first;
            return a.second > b.second;
        }
    };

    vector<string> topKFrequent(vector<string>& words, int k) {
        map<string, int> hash;
        for (auto word : words) hash[word] ++;

        auto cmp = [](const PIS& a, const PIS& b){
            if (a.first != b.first) return a.first < b.first;
            return a.second > b.second;
        };

        //priority_queue<PIS, vector<PIS>, comp> heap;
        priority_queue<PIS, vector<PIS>, decltype(cmp)> heap(cmp);
        for (auto iter : hash) heap.push({iter.second, iter.first});

        vector<string> res;
        while(k --) res.push_back(heap.top().second), heap.pop();

        return res;
    }
};