Leetcode 2351. the first letter that appears twice

1. subject

Give you a string of lowercase English letters s , Please find out and return to first appear two The letter of .

Be careful ：
If a Of The second time Appearance ratio b Of The second time Appear higher in the string , It is considered as a letter a In the letters b Twice before .
s Contains at least one letter that appears twice .

 Example  1：
 Input ：s = "abccbaacz"
 Output ："c"
 explain ：
 Letter  'a'  In subscript  0 、5  and  6  Appears at .
 Letter  'b'  In subscript  1  and  4  Appears at .
 Letter  'c'  In subscript  2 、3  and  7  Appears at .
 Letter  'z'  In subscript  8  Appears at .
 Letter  'c'  Is the first letter that appears twice , Because of all the letters ,'c'  The subscript of the second occurrence is the smallest .

 Example  2：
 Input ：s = "abcdd"
 Output ："d"
 explain ：
 Only letters  'd'  Twice , So back  'd' .
 
 Tips ：
2 <= s.length <= 100
s  It's made up of lowercase letters 
s  Contains at least one repeating letter 

source ： Power button （LeetCode）
link ：https://leetcode.cn/problems/first-letter-to-appear-twice
Copyright belongs to the network . For commercial reprint, please contact the official authority , Non-commercial reprint please indicate the source .

2. Problem solving

• set Store characters that have appeared

class Solution:
    def repeatedCharacter(self, s: str) -> str:
        occ = set()
        for c in s:
            if c not in occ:
                occ.add(c)
            else:
                return c
        return ''

32 ms 14.9 MB Python3

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