# 347. Top K Frequent Elements

Given a non-empty array of integers, return the  k  most frequent elements.

Example 1:

Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]

Example 2:

Input: nums = [1], k = 1
Output: [1]

Note:

• You may assume  k  is always valid, 1 ≤  k  ≤ number of unique elements.
• Your algorithm’s time complexity must be better than O( n  log  n ), where  n  is the array’s size.

class Solution {
public:
vector<int> topKFrequent(vector<int>& nums, int k) {
unordered_map<int, int> m;
priority_queue<pair<int, int>> q;
vector<int> res;
for (auto a : nums) ++m[a];
for (auto it : m) q.push({it.second, it.first});
for (int i = 0; i < k; ++i) {
res.push_back(q.top().second); q.pop();
}
return res;
}
};

class Solution {
public:
vector<int> topKFrequent(vector<int>& nums, int k) {
unordered_map<int, int> m;
vector<vector<int>> bucket(nums.size() + 1);
vector<int> res;
for (auto a : nums) ++m[a];
for (auto it : m) {
bucket[it.second].push_back(it.first);
}
for (int i = nums.size(); i >= 0; --i) {
for (int j = 0; j < bucket[i].size(); ++j) {
res.push_back(bucket[i][j]);
if (res.size() == k) return res;
}
}
return res;
}
};

Github 同步地址：

https://github.com/grandyang/leetcode/issues/347

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https://leetcode.com/problems/top-k-frequent-elements/

https://leetcode.com/problems/top-k-frequent-elements/discuss/81602/Java-O(n)-Solution-Bucket-Sort

https://leetcode.com/problems/top-k-frequent-elements/discuss/81635/3-Java-Solution-using-Array-MaxHeap-TreeMap

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