0347-top-k-frequent-elements.cpp

/*
    Given an integer array nums & an integer k, return the k most frequent elements
    Ex. nums = [1,1,1,2,2,3] k = 2 -> [1,2], nums = [1] k = 1 -> [1]
    
    Heap -> optimize w/ freq map & bucket sort (no freq can be > n), get results from end
*/

// Time: O(n log k)
// Space: O(n + k)

// class Solution {
// public:
//     vector<int> topKFrequent(vector<int>& nums, int k) {
//         unordered_map<int, int> m;
//         for (int i = 0; i < nums.size(); i++) {
//             m[nums[i]]++;
//         }
//         priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> pq;
//         for (auto it = m.begin(); it != m.end(); it++) {
//             pq.push({it->second, it->first});
//             if (pq.size() > k) {
//                 pq.pop();
//             }
//         }
//         vector<int> result;
//         while (!pq.empty()) {
//             result.push_back(pq.top().second);
//             pq.pop();
//         }
//         return result;
//     }
// };

// Time: O(n)
// Space: O(n)

class Solution {
public:
    vector<int> topKFrequent(vector<int>& nums, int k) {
        int n = nums.size();
        
        unordered_map<int, int> m;
        for (int i = 0; i < n; i++) {
            m[nums[i]]++;
        }
        
        vector<vector<int>> buckets(n + 1);
        for (auto it = m.begin(); it != m.end(); it++) {
            buckets[it->second].push_back(it->first);
        }
        
        vector<int> result;
        
        for (int i = n; i >= 0; i--) {
            if (result.size() >= k) {
                break;
            }
            if (!buckets[i].empty()) {
                result.insert(result.end(), buckets[i].begin(), buckets[i].end());
            }
        }
        
        return result;
    }
};