LeetCode451 Sort Characters By Frequency-优快云博客

本文链接：https://blog.youkuaiyun.com/zhangjun62/article/details/96586027

本文介绍了一种算法，该算法接收一个字符串作为输入，并根据字符出现的频率从高到低重新排列字符串中的字符。通过两个不同的实现方式展示了如何完成这一任务，包括使用HashMap进行频率计数并排序以及使用字符数组实现。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

题目：

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:

"tree"

Output:

"eert"

Explanation:

'e' appears twice while 'r' and 't' both appear once.

So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

Example 2:

Input:

"cccaaa"

Output:

"cccaaa"

Explanation:

Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.

Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:

Input:

"Aabb"

Output:

"bbAa"

Explanation:

"bbaA" is also a valid answer, but "Aabb" is incorrect.

Note that 'A' and 'a' are treated as two different characters.

public class SortCharacterByFrequency {
    public String frequencySort(String s) {
        if (s == null)
            return null;
        if (s == "")
            return s;

        HashMap<Character, Integer> charFreqMap = new HashMap<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            charFreqMap.put(c, charFreqMap.getOrDefault(c, 0) + 1);
        }
        ArrayList<Map.Entry<Character, Integer>> list = new ArrayList<>(charFreqMap.entrySet());
        list.sort(new Comparator<Map.Entry<Character, Integer>>(){
            public int compare(Map.Entry<Character, Integer> o1, Map.Entry<Character, Integer> o2) {
                return o2.getValue().compareTo(o1.getValue());
            }
        });
        StringBuffer sb = new StringBuffer();
        for (Map.Entry<Character, Integer> e : list) {
            for (int i = 0; i < e.getValue(); i++) {
                sb.append(e.getKey());
            }
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        System.out.println(new SortCharacterByFrequency().frequencySort("Aabb"));
    }
}

public class SortCharacterByFrequency {
    public String frequencySort(String s) {
        if (s == null)
            return null;
        if (s == "")
            return s;
        char[] charArr = new char[128];
        for(char c :  s.toCharArray())
            charArr[c]++;

        StringBuilder sb = new StringBuilder();


        while(sb.length() < s.length()) {
            char maxChar = 0;
            for(char charCur = 0; charCur < charArr.length; charCur++) {

                if(charArr[charCur] > charArr[maxChar]) {
                    maxChar = charCur;
                }
            }
            while(charArr[maxChar] > 0){
                sb.append(maxChar);
                charArr[maxChar]--;

            }

        }

        return sb.toString();
    }

    public static void main(String[] args) {
        System.out.println(new SortCharacterByFrequency().frequencySort("Aabb"));
    }
}