99 lines
2.3 KiB
C++
99 lines
2.3 KiB
C++
#include <unordered_set>
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#include <vector>
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#include <unordered_map>
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#include <queue>
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/**
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* 347. Top K Frequent Elements
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* Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any order.
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*
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* Complexity analysis:
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* Solution: O(n + k)
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* Solution2: O(n + n * log(k))
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* Solution3: O(n + k * log(n))
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*
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* But "Solution" is the slowest.
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*/
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class Solution {
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public:
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static std::vector<int> topKFrequent(const std::vector<int>& nums, int k) {
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auto* cnt = new std::unordered_set<int>[100000 + 1];
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std::unordered_map<int, int> m;
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int max_cnt = 1;
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for (int x : nums) {
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if (!m.count(x)) {
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m[x] = 1;
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cnt[1].insert(x);
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} else {
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cnt[m[x]].erase(x);
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cnt[++m[x]].insert(x);
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max_cnt = std::max(max_cnt, m[x]);
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}
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}
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std::vector<int> ret;
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for (int i = max_cnt; i > 0 && k > 0; --i) {
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if (cnt[i].size() <= k) {
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k -= cnt[i].size();
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ret.insert(ret.end(), cnt[i].cbegin(), cnt[i].cend());
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} else {
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ret.insert(ret.end(), cnt[i].begin(), std::next(cnt[i].begin(), k));
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}
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}
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delete[] cnt;
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return ret;
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}
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};
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class Solution2 {
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public:
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static std::vector<int> topKFrequent(const std::vector<int>& nums, int k) {
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std::unordered_map<int, int> m;
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for (int x : nums)
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++m[x];
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std::priority_queue<std::pair<int, int>, std::vector<std::pair<int, int>>, std::greater<>> q;
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for (const auto& [idx, cnt] : m) {
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q.push({cnt, idx});
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if (q.size() > k)
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q.pop();
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}
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while (q.size() > k)
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q.pop();
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std::vector<int> ret;
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ret.reserve(k);
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while (!q.empty()) {
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ret.push_back(q.top().second);
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q.pop();
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}
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return ret;
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}
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};
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struct PairSecondCmp {
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bool operator()(const std::pair<int, int> x, const std::pair<int, int> y) {
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return x.second < y.second;
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}
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};
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class Solution3 {
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public:
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static std::vector<int> topKFrequent(const std::vector<int>& nums, int k) {
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std::unordered_map<int, int> m;
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for (int x : nums)
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++m[x];
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std::priority_queue<std::pair<int, int>, std::vector<std::pair<int, int>>, PairSecondCmp> q(m.begin(), m.end());
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std::vector<int> ret;
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ret.reserve(k);
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for (int i = 0; i < k; ++i) {
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ret.push_back(q.top().second);
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q.pop();
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}
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return ret;
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}
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};
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int main() {
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auto ret = Solution3::topKFrequent({5,2,5,3,5,3,1,1,3}, 2);
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return 0;
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}
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