哈希表+排序

方法一：哈希表+数组排序

public String[][] topKstrings (String[] strings, int k) {
        // write code here

        if (strings == null || strings.length == 0){
            return null;
        }

        HashMap<String, Integer> map = new HashMap<>();

        for (String string : strings) {

            map.put(string,map.getOrDefault(string,0)+1);
        }

        String[][] temp = new String[map.keySet().size()][2];
        int i = 0;
        for (String s : map.keySet()) {

            temp[i][0] = map.get(s) + "";
            temp[i++][1] = s;
        }

        Arrays.sort(temp, new Comparator<String[]>() {
            @Override
            public int compare(String[] o1, String[] o2) {

                if (Integer.valueOf(o1[0]) > Integer.valueOf(o2[0])){
                    return -1;
                }else if (Integer.valueOf(o1[0]) == Integer.valueOf(o2[0])){

                    return o1[1].compareTo(o2[1]);
                }else {
                    return 1;
                }
            }
        });

        System.out.println(Arrays.deepToString(temp));

        String[][] res = new String[k][2];

        for (int i1 = 0; i1 < k; i1++) {
            res[i1][0] = temp[i1][1];
            res[i1][1] = temp[i1][0];
        }

        return res;
    }

方法二：哈希表+堆排序
PriorityQueue是java.util包里的一个优先队列，底层是一个小根堆，但是我们可以在创建一个优先队列的时候，传入一个自定义的比较器对象，让其变为大根堆。这样，我们将通过哈希表统计出来的字符串及其出现的频次封装成一个pair对象，然后放入大根堆中，直接让优先队列出队k次即可。

import java.util.*;
import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;
import java.util.PriorityQueue;

public class Solution {
    /**
     * return topK string
     * @param strings string字符串一维数组 strings
     * @param k int整型 the k
     * @return string字符串二维数组
     */
    public String[][] topKstrings (String[] strings, int k) {
        // write code here
    if (strings == null || strings.length == 0) {
            return null;
        }

        HashMap<String, Integer> map = new HashMap<>();

        for (String string : strings) {

            map.put(string, map.getOrDefault(string, 0) + 1);
        }


        PriorityQueue<Pair<String, Integer>> heap = new PriorityQueue<>(new Comparator<Pair<String, Integer>>() {
            @Override
            public int compare(Pair<String, Integer> o1, Pair<String, Integer> o2) {

                if (o1.getValue().compareTo(o2.getValue()) == 1) {
                    return -1;
                } else if (o1.getValue().compareTo(o2.getValue()) == 0) {
                    return o1.getKey().compareTo(o2.getKey());
                } else {
                    return 1;
                }
            }
        });

        for (String s : map.keySet()) {
            heap.add(new Pair<String, Integer>(s,map.get(s)));
        }

        String[][] res = new String[k][2];

        for (int i = 0; i < k; i++) {
            Pair<String, Integer> curNode = heap.poll();
            res[i][0] = curNode.getKey();
            res[i][1] = curNode.getValue()+"";
        }
        return res;
    }
}
    class Pair<K,V>{
        private K key;
        private V value;
        public Pair(K k,V v){
            this.key = k;
            this.value = v;
        }

        public K getKey() {
            return key;
        }

        public V getValue() {
            return value;
        }
    }