【LeetCode】 451. Sort Characters By Frequency
2017-01-07       个评论    来源：LSF_Kevin的博客
我要投稿

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();
}
}
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();
}
}```