Python Leetcode

class Solution:
    def findTheDifference(self, s: str, t: str) -> str:
        d = {}
        for c in s:
            d[c] = d.get(c, 0) + 1
        for c in t:
            d[c] = d.get(c, 0) - 1 
            if d[c] < 0:
                
                return c

class Solution:
    def findTheDifference(self, s: str, t: str) -> str:
        d = {}
        for c in s + t:
            d[c] = d.get(c, 0) + 1
        return [x for x in d if d[x]%2][-1]

class Solution:
    def findTheDifference(self, s: str, t: str) -> str:
        return chr(sum(ord(c) for c in t) - sum(ord(c) for c in s))
        # 利用 Counter (标准库的 collections)
        # return list(Counter(t) - Counter(s))[0]

**知识点：**chr，ord，map，reduce (functools 库)，xor，ascii 码。
使用字符（注意不是字符串）异或运算 ^

• 一个数和 0 做 XOR 运算等于本身：a⊕0 = a
• 一个数和其本身做 XOR 运算等于 0：a⊕a = 0
• XOR 运算满足交换律和结合律：a⊕b⊕a = (a⊕a)⊕b = 0⊕b = b

class Solution:
    def findTheDifference(self, s: str, t: str) -> str:
        res = 0
        for c in s + t:
            res ^= ord(c)
        
        return chr(res)

        # return chr(reduce(xor, map(ord, s + t)))

class Solution:
    def fractionToDecimal(self, numerator: int, denominator: int) -> str:
        flag = '' if numerator * denominator >= 0 else '-'
        n, den = abs(numerator), abs(denominator)
        d, r = divmod(n, den)
               
        # res = str(d) + '.' if r else str(d)   
        # i, s = len(res), {}
        pre = str(d) + '.' if r else str(d)
        i, res, s = 0, '', {}

        while r:
            d, r = divmod(r * 10, den)  
            res += str(d)
            i += 1
        
            if r in s:
                res = res[:s[r]] + "(" + res[s[r]:] + ")"        
                break
            else:
                s[r] = i

        # return flag + res
        return flag + pre + res