leetcode 探索 查找表类算法

1、给定一个字符串，请将字符串里的字符按照出现的频率降序排列。

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: 三数之和。

def threeSum(nums):
    res = []
    nums.sort()
    n = len(nums)
    #如果数组长度小于3，也就不存在三数之和，返回[]
    if n<3:
        return res
    #遍历数组：
    for i in range(n):
        #数组已完成排序，i 之后的元素不存在三数之和=0的情况
        if nums[i]>0:
            break
            #跳过相邻的重复元素。
        if (i>0 and nums[i]==nums[i-1]):
            continue
        #初始化指针：令 i<l<r，然后进入循环。
        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))

 4、给定一个字符串数组，将字母异位词组合在一起。字母异位词指字母相同，但排列不同的字符串。

输入: ["eat", "tea", "tan", "ate", "nat", "bat"] 输出：[ ["ate","eat","tea"],["nat","tan"],["bat"]].
算法：将字母异位词的组成字母以元组的形式构成键(key)，字母异位词以列表形式构成值(value).例如：dic = {(a,e,t):["eat","tea","ate"]}

def groupAnagrams(strs):
    dic = {}
    for i in strs:
        res = tuple(sorted(i))
        if res in dic.keys():
            dic[res].append(i)
        else:
            dic[res] = [i]
    return list(dic.values())

strs = ["eat", "tea", "tan", "ate", "nat", "bat"]
print(groupAnagrams(strs))