347 Top K Frequent Elements
1. Question
Given a non-empty array of integers, return thekmost frequent elements.
For example,
Given[1,1,1,2,2,3]
and k = 2, return[1,2]
.
Note:
You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
Your algorithm's time complexity must be better than O(nlogn), where n is the array's size.
2. Implementation
(1) Bucket Sort
class Solution {
public List<Integer> topKFrequent(int[] nums, int k) {
List<Integer> res = new ArrayList<>();
if (nums.length == 0 || k <= 0) {
return res;
}
Map<Integer, Integer> map = new HashMap<>();
for (int num : nums) {
map.put(num, map.getOrDefault(num, 0) + 1);
}
List<Integer>[] bucket = new List[nums.length + 1];
for (int key : map.keySet()) {
int freq = map.get(key);
if (bucket[freq] == null) {
bucket[freq] = new ArrayList<>();
}
bucket[freq].add(key);
}
for (int i = nums.length; i >= 0 && res.size() < k; i--) {
if (bucket[i] != null) {
res.addAll(bucket[i]);
}
}
return res;
}
}
(2) Heap
class Solution {
public List<Integer> topKFrequent(int[] nums, int k) {
List<Integer> res = new ArrayList<>();
if (nums == null || nums.length == 0) {
return res;
}
Map<Integer, Integer> map = new HashMap<>();
for (int num : nums) {
map.put(num, map.getOrDefault(num, 0) + 1);
}
PriorityQueue<NumInfo> minHeap = new PriorityQueue<>();
for (int key : map.keySet()) {
minHeap.add(new NumInfo(key, map.get(key)));
if (minHeap.size() > k) {
minHeap.remove();
}
}
while (!minHeap.isEmpty()) {
res.add(0, minHeap.remove().val);
}
return res;
}
class NumInfo implements Comparable<NumInfo> {
int val;
int freq;
public NumInfo(int val, int freq) {
this.val = val;
this.freq = freq;
}
public int compareTo(NumInfo that) {
return this.freq - that.freq;
}
}
}
3. Time & Space Complexity
Bucket Sort: 时间复杂度O(n), 空间复杂度O(n)
Heap: 时间复杂度O(n + nlogk), 空间复杂度O(n)
Last updated
Was this helpful?