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【LeetCode】 451. Sort Characters By Frequency

2017-01-07 09:25:00      个评论    来源:LSF_Kevin的博客  
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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 Solution {
    public String frequencySort(String s) {
        char[] array = s.toCharArray();
        HashMap map = new HashMap();
        int max = 1;
        for (int i = 0; i < array.length; i++) {
            Integer temp = map.get(array[i]);
            if (temp == null) {
                map.put(array[i], 1);
            } else {
                map.put(array[i], temp + 1);
                max = Math.max(max, temp + 1);
            }
        }
        List[] list = new List[max + 1];
        for (Character c : map.keySet()) {
            int num = map.get(c);
            if (list[num] == null) {
                list[num] = new ArrayList();
            }
            list[num].add(c);
        }
        StringBuilder sb = new StringBuilder();
        for (int i = list.length - 1; i >= 0; i--) {
            List subList = list[i];
            if (subList != null) {
                for (Character c : subList) {
                    for (int j = 0; j < i; j++) {
                        sb.append(c);
                    }
                }
            }
        }
        return sb.toString();
    }
}


 


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