题目:
Given two strings
s
andt
, returntrue
ift
is an anagram ofs
, andfalse
otherwise.An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.
这里 Anagram 的定义就是可以通过重新排序获得的单词,如 cat
可以获得 act
,这种,所以满足的条件有两个:
我刚开始的写法是这样的:
class Solution:
def isAnagram(self, s: str, t: str) -> bool:
# s being an anagram of t means:
# 1. same length
# 2. same # of chars
if len(s) != len(t):
return False
# create a dict to store key val frequency
char_dict = {}
for l in s:
if l in char_dict:
char_dict[l] = char_dict[l] + 1
else:
char_dict[l] = 1
# find if all chars appeared in the dict
for l in t:
if l not in char_dict:
return False
if char_dict[l] == 1:
del char_dict[l]
else:
char_dict[l] = char_dict[l] - 1
return True
第一个检查字符串的长度,二者不一致就可以直接返回 false
然后进入两个循环,第一个循环创建一个 dict,进行字符和频率的 mapping,第二个循环检查当前字符是否出现在 dict 中,如果没有返回 false,如果有,那么出现的频率减少
如果第二个循环走完了,就代表两个字符的出现的字符串频率一致,也就可以返回 true
写完后我参考了一下别人的写法,发现一个更优雅的答案:
class Solution:
def isAnagram(self, s: str, t: str) -> bool:
if len(s) != len(t):
return False
countS, countT = {}, {}
for i in range(len(s)):
countS[s[i]] = 1 + countS.get(s[i], 0)
countT[t[i]] = 1 + countT.get(t[i], 0)
return countS == countT
我找了一下,dict 中的 get
第二个参数为默认值,即当当前值不存在于 dict 中使用的值,这样可以减少一重 if/else
比较两个 dict 这个我找了下资料, Comparing two dictionaries and checking how many (key, value) pairs are equal
中提到了 python 的文档中 Mapping Types — dict¶ 用了这个案例:
The following examples all return a dictionary equal to
{"one": 1, "two": 2, "three": 3}
:>>> a = dict(one=1, two=2, three=3) >>> b = {'one': 1, 'two': 2, 'three': 3} >>> c = dict(zip(['one', 'two', 'three'], [1, 2, 3])) >>> d = dict([('two', 2), ('one', 1), ('three', 3)]) >>> e = dict({'three': 3, 'one': 1, 'two': 2}) >>> a == b == c == d == e True
Providing keyword arguments as in the first example only works for keys that are valid Python identifiers. Otherwise, any valid keys can be used.
另一个 post What does the == operator actually do on a Python dictionary? 中提到,==
中有提到,==
的结果为 true 的情况 sorted(key, value) 结果完全一致,第一条回答找了一个 python 的源码实现,说内部进行递归调用判断 dict 中每个元素都是相通的,不过目前代码的 reference 消失了
总结就是,如果两个 dict 的 key 相同,每个 key 的 value 也相同,就可以用 ==