347. Top K Frequent Elements

https://leetcode.com/problems/top-k-frequent-elements/discuss/126241/java

先记录次数 然后按次数放进数组 然后从后往前取出

 

 

 

 

class Solution {
    public List<Integer> topKFrequent(int[] nums, int k) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for(int num : nums) {
            map.put(num, map.getOrDefault(num, 0) + 1);
        }
        
        Set<Integer> set = map.keySet();
        List<Integer>[] record = new List[nums.length + 1]; //注意新建方法没有<>
        for(int key : set) {
            int freq = map.get(key);
            if(record[freq] == null) {
                record[freq] = new ArrayList<>(); //注意一下要新建
            }
            record[map.get(key)].add(key);
        }
        
        List<Integer> res = new ArrayList<>();
        for(int i = record.length - 1; i >= 0; i--) {
            if(record[i] != null) {
                for(int j = 0; j < record[i].size(); j++) {
                    if(res.size() < k) {
                        res.add(record[i].get(j));
                    }else {
                        break;
                    }
                }    
            }
            
        }
        return res;   
    }
}