常用的排序算法------练习3

1. 题目

2. 思路和题解

这道题和昨天那道347题一样，也是用桶排序来做。主要步骤如下：

1. 遍历字符串，统计每个字符出现的频率，同时记录最高频率 maxFreq
2. 创建桶，存储从 1 到 maxFreq 的每个出现频率的字符
3. 按照出现频率从大到小的顺序遍历桶，对于每个出现频率，获得对应的字符，然后将每个字符按照出现频率拼接到排序后的字符串

所以整体的代码如下：

class Solution {
    public String frequencySort(String s) {
        Map<Character, Integer> map = new HashMap<Character, Integer>();
        int maxFreq = 0;
        int length = s.length();
        for (int i = 0; i < length; i++) {
            char c = s.charAt(i);
            int frequency = map.getOrDefault(c, 0) + 1;
            map.put(c, frequency);
            maxFreq = Math.max(maxFreq, frequency);
        }
        StringBuffer[] buckets = new StringBuffer[maxFreq + 1];
        for (int i = 0; i <= maxFreq; i++) {
            buckets[i] = new StringBuffer();
        }
        for (Map.Entry<Character, Integer> entry : map.entrySet()) {
            char c = entry.getKey();
            int frequency = entry.getValue();
            buckets[frequency].append(c);
        }
        StringBuffer sb = new StringBuffer();
        for (int i = maxFreq; i > 0; i--) {
            StringBuffer bucket = buckets[i];
            int size = bucket.length();
            for (int j = 0; j < size; j++) {
                for (int k = 0; k < i; k++) {
                    sb.append(bucket.charAt(j));
                }
            }
        }
        return sb.toString();
    }
}