Most Frequent Character

Given a string s, find the most frequent character in the string. In case of a tie, return the lexicographically smallest character.

If the input string is empty, output None.

The problem requires you to process the string and compute the frequency of each character. You then determine which character has the highest frequency. In the event of a tie, the character with the smaller ASCII value (i.e. lexicographically smaller) should be chosen.

The formula for frequency analysis is given as follows:

\[ \text{Frequency}(c) = \sum_{i=1}^{n} \mathbf{1}_{\{s_i = c\}}, \] where \(\mathbf{1}\) is the indicator function and \(s_i\) is the i-th character of the string.

inputFormat

The input consists of a single line containing the string s. Note that s may be empty.

outputFormat

Output a single line which contains the most frequent character in the string. If the string is empty, output None.

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