2021-07-05:每日5题：哈希表：1，217，594，128

1. two sum

217:

注意：

hashmap = defaultdict(int)  #初始值为0

还有 dict.get(i,0) +1，这样也可以创建key,value

defaultdict(float)  #初始值为0.0

defaultdict(str) #初始值为“”

class Solution:
    def containsDuplicate(self, nums: List[int]) -> bool:
        hashmap = defaultdict(int)
        for num in nums:
            if hashmap[num] == 1:
                return True
            hashmap[num] = 1
        return False

594

解题思路

利用哈希表存储数组中元素出现的次数
遍历哈希表，如果当前元素+1也在哈希表中，那么计算两者次数之和，保留最大值

class Solution:
    def findLHS(self, nums: List[int]) -> int:
        hashmap = defaultdict(int)
        max = 0
        for num in nums:
            hashmap[num] += 1
        for i in hashmap:
            if hashmap.get(i) is not None and hashmap.get(i+1) is not None:
                if hashmap.get(i) + hashmap.get(i+1) > max:
                    max = hashmap.get(i) + hashmap.get(i+1)
        return max

注意如何遍历字典：

python遍历字典一共有四种方式

　D={‘a’：1，‘b’：2，’b‘：3，’d‘：4}

1. 遍历key值

for key in a:

     pritn(key+':'+a[key])

for key in a.keys():

      print(key+':'+a[key])

2.遍历value值

for value in a.values():

    print(value)

3.遍历字典项

for kv in a.items():

    print(kv)

（‘a’,'1'）(...)

4.遍历字典键值

for key,value in a.items():

    print(key+':'+value)

for (key,value) in a.items():

    print(key+':'+value)

class Solution:
    def findLHS(self, nums: List[int]) -> int:
        dicts={}
        res = 0
        for num in nums:
            dicts[num] = dicts.get(num,0) +1
        for key in dicts:
            if key +1 in dicts:
                res = max(res, dicts[key]+dicts[key+1])
        return res
                

如果没有时间要求：

Nlogn,直接先排序

class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        if not nums:
            return 0
        nums = sorted(nums)
        print(nums)
        max_l = 1
        l = 1
        prev = nums[0]
        for num in nums[1:]:
            if num == prev:
                continue
            elif num == prev+1:
                l += 1
            else:  
                l = 1
            prev = num
            max_l = max(max_l,l)
        return max_l
        

要求时间为O(n)

class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        if not nums:
            return 0
        nums= set(nums)
        l = 1
        max_l = 1
        for num in nums:
            if num-1 not in nums:
                l = 1
                while num+1 in nums:
                    l+=1
                    num = num+1
                    max_l = max(l,max_l)              
        return max_l