給一個字串，要以字母的出現次數重新排列。
首先要有可以統計字母的方式，用 hash table 可以很容易記錄哪些字的有出現與其次數。
python 有 collections.Counter 可以快速轉換。
Python
class Solution:
def frequencySort(self, s: str) -> str:
counter = Counter(s)
result = ""
for each in sorted(counter.items(), key = lambda x: x[1], reverse = True):
result += each[0] * each[1]
return result
C#
public class Solution {
public string FrequencySort(string s) {
var counter = new Dictionary();
foreach(char c in s)
{
if(counter.ContainsKey(c))
{
counter[c] += 1;
}
else
{
counter[c] = 1;
}
}
var result = new StringBuilder();
foreach(var item in counter.OrderByDescending(x=>x.Value))
{
result.Append(item.Key, item.Value);
}
return result.ToString();
}
}