PROGRAMMING:Find the most frequent character in a string

Find the most frequent character in a string.
###Input format:
Given a string, the length of the string is not greater than 10 ^ 6, case insensitive, string may contain 'a' -'z ','a' -'z ',' '.
###Output format:
Output the characters that appear the most times (if they are letters, output lowercase letters), and separate them with a space. If there are characters that appear the same times, output smaller characters. There are no extra spaces and line breaks at the end of the line
###Input example:
```in
abcdefghijklmnopqrstuvwxyzA
```
###Output example:
```out
a 2
```






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