Top K Frequent Elements

Given a non-empty array of integers, return the k most 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(n logn), wheren is the array's size.

这个题不难，就是复习下treemap的用法。

下面的工具类可以将Hashmap排序。将Hashmap转变成LinkedHashMap。

class MapUtil  {      public static <K, V extends Comparable<? super V>> Map<K, V>           sortByValue( Map<K, V> map )      {          List<Map.Entry<K, V>> list =              new LinkedList<Map.Entry<K, V>>( map.entrySet() );          Collections.sort( list, new Comparator<Map.Entry<K, V>>()          {              public int compare( Map.Entry<K, V> o1, Map.Entry<K, V> o2 )              {                  return (o1.getValue()).compareTo( o2.getValue() );              }          } );            Map<K, V> result = new LinkedHashMap<K, V>();          for (Map.Entry<K, V> entry : list)          {              result.put( entry.getKey(), entry.getValue() );          }          return result;      }  }  

AC代码：（当然可以用上面的方法做）

public class Solution {    class myComparator implements Comparator<Integer>{Map<Integer, Integer> map;public myComparator(Map<Integer, Integer> map) {this.map=map;}@Overridepublic int compare(Integer o1, Integer o2) {//大于，-1是降序，1是升序if(map.get(o1)>map.get(o2))return -1;return 1;}}public List<Integer> topKFrequent(int[] nums, int k) {List<Integer> result=new ArrayList<Integer>();        Map<Integer, Integer> map=new TreeMap<Integer, Integer>();        for(int i=0;i<nums.length;i++){        if(!map.containsKey(nums[i])){        map.put(nums[i], 1);        }else{        map.put(nums[i], map.get(nums[i])+1);        }        }        Comparator<Integer> myComparator=new myComparator(map);        Map<Integer, Integer> treemap=new TreeMap<Integer, Integer>(myComparator);        treemap.putAll(map);        for (Integer key : treemap.keySet()) {result.add(key);k--;if(k==0){break;}}        return result;    }}