451. Sort Characters By Frequency

题目大意：

按照字符串中的字符出现次数排序，注意大写和小写字符要区分开。一道有点麻烦的题，用一个Map存储字符及出现的次数，然后重写comparator对Map进行排序。

Java代码如下：

import java.util.*;
import java.util.Map.Entry;

public class Solution {
    public List<Map.Entry<Character, Integer>> getCount(String str) {
        Map<Character, Integer> map = new HashMap<Character, Integer>();
        int len = str.length();
        for(int i = 0; i < str.length(); i++) {
            char s = str.charAt(i);
            if(map.containsKey(s)) {
                map.put(s, map.get(s) + 1);
            }else {
                map.put(s, 1);
            }
        }

        List<Map.Entry<Character, Integer>> list = 
                new ArrayList<Map.Entry<Character, Integer>>(map.entrySet());
        Collections.sort(list, new Comparator<Map.Entry<Character, Integer>>(){
            @Override
            public int compare(Entry<Character, Integer> arg0, Entry<Character, Integer> arg1) {
                return arg1.getValue() - arg0.getValue();   
            }
        });
        return list;
    }


    public String frequencySort(String s) {
        List<Map.Entry<Character, Integer>> list = getCount(s);
        StringBuilder sb = new StringBuilder();
        for(Entry<Character, Integer> entry : list) {
            System.out.println(entry.getKey() + " : " + entry.getValue());
            Character c = entry.getKey();
            for(int i = 0; i < entry.getValue(); i++) {
                sb.append(c);
            }
        }
        return sb.toString();
    }

}