题目描述
You are given a string S consisting of lowercase English letters.
Find the lexicographically (alphabetically) smallest lowercase English letter that does not occur in S.
If every lowercase English letter occurs in S, print None instead.
-----Constraints-----
- $1 \leq |S| \leq 10^5 (|S|$ is the length of string S.)
- S consists of lowercase English letters.
-----Input-----
Input is given from Standard Input in the following format:
S
-----Output-----
Print the lexicographically smallest lowercase English letter that does not occur in S.
If every lowercase English letter occurs in S, print None instead.
-----Sample Input-----
atcoderregularcontest
-----Sample Output-----
b
The string atcoderregularcontest contains a, but does not contain b.
输入格式
见题面
输出格式
见题面