1、給定一個字串，請將字串裡的字元按照出現的頻率降序排列。

import collections
def frequencySort(s):
    dic = dict(collections.Counter(s))
    res = sorted(dic.items(),key = lambda item:item[1],reverse = True)
    temp = ""
    for i in res:
        temp +=i[0]*i[1]
    return temp

s = "tree"
print(frequencySort(s))

3: 三數之和。

def
 
 threeSum(nums):
    res = []
    nums.sort()
    n = len(nums)
    #如果陣列長度小於3，也就不存在三數之和，返回[]
    if n<3:
        return res
    #遍歷陣列：
    for i in range(n):
        #陣列已完成排序，i 之後的元素不存在三數之和=0的情況
        if nums[i]>0:
            return res
        #跳過相鄰的重複元素。
        if (i>0 and nums[i]==nums[i-1]):
            
 
continue
        #初始化指標：令 i<l<r，然後進入迴圈。
        l = i+1
        r = n-1
        while l<r:
            sum = nums[i]+nums[l]+nums[r]
            if sum==0:
                res.append([nums[i],nums[l],nums[r]])
                #去重
                while (l<r and nums[l]==nums[l+1]):
                    l 
 
= l+1
                while (l<r and nums[r]==nums[r-1]):
                    r = r-1

                l = l+1
                r = r-1

            elif sum>0:
                r = r-1
            else:
                l = l+1
    return res


nums = [-1, 0, 1, 2, -1, -4]
print(threeSum(nums))