Leetcode刷题 - Top K 系列

347. Top K Frequent Elements

class Solution {
public:
    vector<int> topKFrequent(vector<int>& nums, int k) {
        // hash map 储存frequency
        unordered_map<int, int> map;
        for (int num : nums)
            map[num]++;
        
        vector<int> ans;
        // use priority queue to build max heap
        priority_queue<pair<int,int>> pq; 
        // 迭代map加入堆中，寻找最大频率对应的数
        for(auto iter = map.begin(); iter != map.end(); iter ++){
            pq.push(make_pair(iter->second, iter->first));
            if (pq.size() > map.size() - k){
                ans.push_back(pq.top().second);
                // 加入ans中，弹出堆顶元素
                pq.pop();
            }
        }
        return ans;
    }
};

 

692. Top K Frequent Words 

class Solution {
public:
    vector<string> topKFrequent(vector<string>& words, int k) {
        //初始化hashmap
        unordered_map<string, int> map;
        for (string s : words)
            map[s]++;
        
        vector<string> ans;
        //初始化最小堆
        //自定义 compare
        // 若次数相同，则按字母来排序
        // return true if a is considered to go before b
        auto comp = [](pair<int, string>& a, pair<int, string>& b){ return a.first == b.first ? a.second < b.second : a.first > b.first; };
        priority_queue<pair<int, string>, vector<pair<int, string> >, decltype(comp)>pq(comp);
        // 调整最小堆
        for (auto iter = map.begin(); iter != map.end(); iter++){
            pq.push(make_pair(iter->second, iter->first));
            if (pq.size () > k) pq.pop(); 
        }
        // 加入结果
        while(!pq.empty()) {
            ans.push_back(pq.top().second); 
            pq.pop();
        }
        //翻转结果
        reverse(ans.begin(), ans.end());
        return ans;
    }
};

　　

215. Kth Largest Element in an Array

class Solution {
public:
    int findKthLargest(vector<int>& nums, int k) {
        // 建一个大小为2的最小堆
        priority_queue<int, vector<int>, greater<int>> pq;
        for (int n : nums){
            pq.push(n);
            if (pq.size() > k) pq.pop();
        }
        
        return pq.top();
    }
};

 

973. K Closest Points to Origin

class Solution {
public:
    vector<vector<int>> kClosest(vector<vector<int>>& points, int K) {
        //建立一个长度为k的最小堆
        auto comp = [](vector<int>& a, vector<int>&b){return a[0]*a[0] + a[1]*a[1] < b[0]*b[0] + b[1]*b[1];};
        priority_queue<vector<int>, vector<vector<int>>, decltype(comp)> pq (comp);
        //添加元素，调整堆
        for(vector<int> temp : points){
            pq.push(temp);
            if(pq.size() > K){
                pq.pop();
            }
        }
        //输出结果
        vector<vector<int>> ans;
        while(!pq.empty()){
            ans.push_back(pq.top());
            pq.pop();
        }
        return ans;
    }
};

　　

451. Sort Characters By Frequency

class Solution {
public:
    string frequencySort(string s) {
        // hashmap to store the frequency
        unordered_map<char, int> map;
        for (char ch:s)
            map[ch]++;
        
        //建立一个最大堆
        priority_queue<pair<int, char>> pq;
        for (auto m : map)
            pq.push({m.second, m.first});
        
        // 输出结果
        string result = "";
        while(!pq.empty()){
            /********************************************
            * string (size_t n, char c) :frequency times the character
            **********************************************/
            result += string(pq.top().first, pq.top().second);
            // result.push_back(pq.top().first);
            pq.pop();
        }
        return result;
    }
};

　　

总结

1. 识别最大K个元素模式：

• 如果你需要求最大/最小/最频繁的前K个元素
• 如果你需要通过排序去找一个特定的数

2. 步骤

• 初始化堆：priority_queue
• 确定堆的属性：STL默认为最大堆, 并根据int type进行排序。也可以用compare自行写判断条件

auto comp = [](pair<int, string>& a, pair<int, string>& b){ return a.first == b.first ? a.second < b.second : a.first > b.first; };
priority_queue<pair<int, string>, vector<pair<int, string> >, decltype(comp)>pq(comp);　

• 将数据插入堆中，调整堆

 

参考资料

1. https://leetcode.com/problems/top-k-frequent-elements/
2. https://leetcode.com/problems/top-k-frequent-words/
3. https://leetcode.com/problems/kth-largest-element-in-an-array/
4. https://leetcode.com/problems/k-closest-points-to-origin/
5. https://leetcode.com/problems/sort-characters-by-frequency/