The first letter of leetcode simple question appears twice

subject

Give you a string of lowercase English letters s , Please find out and return to the first occurrence 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）

Their thinking

   This is a simple character frequency statistics problem , You can start to count the character frequency first. Once you find a qualified character in the statistical process , You can return the result .

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