Sort by character occurrence frequency 451

• Question no ： Power button 451
• Knowledge point ： Hashtable , Count
• Goal completion ：25/150
• summary
  stem ：
  

Ideas ：

• 1. First save the letter in the string and the number of times the letter appears in the dictionary hashmap in
• 2. then hashmap The letter stored in and the number of times the letter appears are transferred to the list res in , It is convenient to sort the number of occurrences from more to less , After finishing the sequence ,res According to the number of letters in the order from high to low
• 3. Re traversal res, Put each letter y[0] stay ans The number of times the letter appears is appended to the y[1] Time

class Solution:
    def frequencySort(self, s: str) -> str:
        hashmap = {
    }
        for x in s:
            if x in hashmap:
                hashmap[x] += 1
            else:
                hashmap[x] = 1
        res = []
        for key, value in hashmap.items():
            res.append([key, value])
        res.sort(key=lambda arr:arr[1], reverse=True)
        
        ans = ''
        for y in res:
            for i in range(y[1]):
                ans += (y[0])
        return ans