剑指 Offer II 004.

原题

给你一个整数数组 nums ，除某个元素仅出现 一次 外，其余每个元素都恰出现 三次 。请你找出并返回那个只出现了一次的元素。

解法1【哈希表】

示例

输入：nums = [0, 1, 0, 1, 0, 1, 99]

输出：99

python中的dict用的就是哈希表

class Solution:
    def singleNumber(self, nums: List[int]) -> int:
        num_dict = {}
        for num in nums:
            count = num_dict.get(num)
            if count is None:
                num_dict[num] = 1

            elif count == 1:
                num_dict[num] += 1

            elif count == 2:
                num_dict.pop(num)

        return [key for key in num_dict.keys()][0]

字典的操作

The dict() Constructor

thisdict = dict(name = "John", age = 36, country = "Norway")
print(thisdict)

Check if Key Exists

for key in thisdict:  # 直接用for循环得到键
    print(key in thisdict)  # 直接用in判断键是否在字典中

Update Dictionary

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.update({"year": 2020, "new":2022})
# 老的key会更新，新的key会添加
# {'brand': 'Ford', 'model': 'Mustang', 'year': 2020, 'new': 2022}

Method	Description
clear()	Removes all the elements from the dictionary
copy()	Returns a copy of the dictionary
fromkeys()	Returns a dictionary with the specified keys and value
get(key, default=None)	Returns the value of the specified key
items()	Returns a list containing a tuple for each key value pair
keys()	Returns a list containing the dictionary's keys
pop()	Removes the element with the specified key
popitem()	Removes the last inserted key-value pair
setdefault(key, default=None)	Returns the value of the specified key. If the key does not exist: insert the key, with the specified value
update()	Updates the dictionary with the specified key-value pairs
values()	Returns a list of all the values in the dictionary