LeeCode : 451. Sort Characters By Frequency 按频率给字符排序

我就是我 2021-06-24 16:10 326阅读 0赞

**试题：**  
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.  
**代码：**  
根据字符的某种信息来给字符排序。  
桶排序：先统计频率，然后生成桶，按照频率放置到各个桶，然后生成排序。

class Solution {
        public String frequencySort(String s) {
            Map<Character, Integer> counter = new HashMap<>();
            for (char c : s.toCharArray())
                counter.put(c, counter.getOrDefault(c, 0) + 1);
    
            List<Character>[] bucket = new ArrayList[s.length() + 1];
            for (char c : counter.keySet()) {
                int count = counter.get(c);
                if (bucket[count] == null) {
                    bucket[count] = new ArrayList<>();
                }
                bucket[count].add(c);
            }
            StringBuilder str = new StringBuilder();
            for (int i = bucket.length - 1; i > 0; i--) {
                if (bucket[i] == null) {
                    continue;
                }
                for (char c : bucket[i]) {
                    for (int j = 0; j < i; j++) {
                        str.append(c);
                    }
                }
            }
            return str.toString();
        }
    }